Scholarly article on topic 'Theoretical study of the electronic spectra of neutral and cationic NpO and NpO2'

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Academic research paper on topic "Theoretical study of the electronic spectra of neutral and cationic NpO and NpO2"

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The Journal of Chemical Physics

Theoretical study of the electronic spectra of neutral and cationic NpO and NpO2

Attila Kovacs and Ivan Infante

Citation: The Journal of Chemical Physics 143, 074305 (2015); doi: 10.1063/1.4928588 View online: http://dx.doi.org/10.1063/14928588

View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/143/7?ver=pdfcov Published by the AIP Publishing

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Theoretical study of the electronic spectra of neutral and cationic NpO and NpO2

Attila Kovacs1 and Ivan Infante2

1 European Commission, Joint Research Centre, Institute for Transuranium Elements, P.O. Box 2340, 76125 Karlsruhe, Germany

2Department of Theoretical Chemistry, Faculty of Sciences, Vrije Universiteit Amsterdam, De Boelelaan 1083, 1081 HVAmsterdam, The Netherlands

(Received 18 May 2015; accepted 3 August 2015; published online 19 August 2015)

The electronic spectra of neutral NpO and NpO2 as well as of their mono- (NpO+, NpO2+) and dications (NpO2+, NpO22+) were studied using multiconfigurational relativistic quantum chemical calculations at the complete active space self-consistent field/CASPT2 level of theory taking into account spin-orbit coupling. The active space included 16 orbitals: all the 7s, 6d, and 5f orbitals of neptunium together with selected orbitals of oxygen. The vertical excitation energies on the ground state geometries have been computed up to ca. 35 000 cm-1. The gas-phase electronic spectra were evaluated on the basis of the computed Einstein coefficients at 298 K and 3000 K. The computed vertical transition energies show good agreement with previous condensed-phase results on NpO2+ andNpO22+. © 2015 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License. [http://dx.doi.org/10.1063/L4928588]

I. INTRODUCTION

Neptunium oxides are present in notable amount in spent nuclear fuel, for which several reprocessing and recycling technologies have been developed, including oxide-based transmutation fuels.1-6 However, these technologies can gain from optimization of the conditions, requiring an accurate knowledge of the molecular properties of all actinide compounds present in the fuel.

In contrast to the detailed experimental description of the solid neptunium oxides,7 their molecular properties are less explored. To obtain them requires difficult and expensive gas-phase experiments which were performed hitherto for the ionization energies (NpO8-10 and NpO29,10) and dissociation energies of both the neutral oxides and their cations.10,11 Structural and vibrational data on the NpO and NpO2 molecules are available from quantum chemical calculations only.12,13

Experimental studies of the electronic structure were performed for the ions NpO2+ and NpO22+ and only in condensed phases. Early absorption spectra of Np oxides (and also other actinide compounds) were reviewed in Ref. 14 and 15. Both ions were investigated by UV/VIS absorption spectroscopy in aqueous acidic solutions,16-18 accompanied by separate studies on NpO2+19 and on NpO22+.20 The absorption spectrum of NpO22+ was also recorded in the form doped into Cs2UO2Cl4 and CsUO2(NO3)3 crystals.21-25 These latter spectra were similar to the solution ones implying that the transition energies are not much affected by the environment. The ground electronic states could be well determined on the basis of the above experiments: they are the 3H4g state for NpO2+26 and the 2$5/2u state for NpO22+.23,24,26

The electronic spectra of the solutions were first assigned using semi-empirical ligand-field theory (NpO2+27 and

NpO22+ 28) and were revised later by quantum chemical calculations.19,25,29-35 Matsika et al.19,25,29 used configuration interaction methods in conjunction with relativistic effective core potentials that incorporate also spin-orbit coupling36 (SO-MRCI). In one of these studies,19 coordination models with five explicit coordinating water molecules and chloride ions, respectively, were tested. Infante et al.30 used for the free ions the more advanced intermediate Hamiltonian Fock-space coupled-cluster (IHFSCC) method in conjunction with the Dirac-Coulomb Hamiltonian which accounted for relativistic effects including spin-orbit coupling as well. Recently, Danilo et al. applied SO-CASPT2 and SO-MRCI calculations using a small active space for modelling the free and hydrated NpO+ (the latter as [NpO2(H2O)5]+) ions.32 The ground and the four lowest-energy excited states of NpO22+ and of the NpO2Cl42- complex ion (model for the Cs2NpO2Cl4 solidphase spectra) were investigated by Gomes et al. using IHFSCC.31 They considered further environmental effects on NpO2Cl24- by means of density functional theory (DFT) embedding. The hitherto most complete assessment of the electronic spectrum of Cs2NpO2Cl4 at an adequate level of theory was performed by Su et al.33 who calculated both the vertical and adiabatic transitions of NpO22+ and NpO2Cl42- up to 21 000 cm-1 using the SO-RASPT2 method. The electronic structure and magnetic properties of NpO22+ and a few complexes: NpO2Cl42-, [NpO2(NO3)3]-, and [NpO2(CO3)3]4-were studied by Gendron et al.34,35 using a combination of theoretical (among them SO-CASPT2) methods. The studies included a detailed analysis of SO coupling involving the first four electronic states of the given molecules.

In other recent studies, only the ground-state electronic structure and molecular properties were investigated: Liao et al. computed NpO2 and NpO22+ using relativistic DFT calculations.37 They reported the electronic ground states of

0021-9606/2015/143(7)/074305/12 143,074305-1 © Author(s) 2015

NpO2 and NpO22+ (that of NpO2 erroneously as 4Sg) as well as the first and second ionization energies of NpO2. Several computational studies were performed on neutral and ionic neptunium mono- and dioxides at SO-CASPT2 and various DFT levels resulting in theoretical ionization energies,12 38 vibrational frequencies,13 and dissociation energies.3839 The hydration and oxidation reactions of NpO2+ in the gaseous phase were studied by DFT assisting electrospray experiments,40 while NpO22+ served as model compound (focusing on the bond distance and vibrational frequencies) in a benchmark study of two-component relativistic DFT methods.41

The goal of the present study is the systematic evaluation of the gas-phase electronic spectra of the neutral and ionic NpO and NpO2 species by means of multireference relativistic ab initio calculations on the basis of our previous studies on the electronic ground states of these species.12,38 Other important results from the present calculations are the energies of the excited states, required for a reliable evaluation of the thermodynamic functions of the gaseous oxides. Such thermodynamic data are utilized in developing safety procedures in nuclear industry.

II. COMPUTATIONAL DETAILS

The calculations were performed using the software MOLCAS 7.4.42 The complete active space self-consistent field (CASSCF) method43 was used to generate molecular orbitals and reference functions for subsequent multiconfigu-rational second-order perturbation theory calculations of the dynamic correlation energy (CASPT2).44,45 In the case of the monoxides, the active space consisted of 7s, 6d, and 5f orbitals of the actinide atoms as well as of the 2p orbitals of oxygen. This means 16 orbitals occupied by 11, 10, and 9 electrons for NpO, NpO+, and NpO2+, respectively. An analogous construction of the active space of the dioxides would result in 19 orbitals occupied by 13-15 electrons in the studied dioxide species, which is nowadays computationally unfeasible due to the large amount of configuration state functions. Therefore, we used a truncated space of 14 orbitals, omitting the two lowest ng bonding and the corresponding antibonding orbitals and one og* antibonding orbital. This active space included 11,10, and 9 electrons for NpO2, NpO2+, and NpO22+, respectively. We neglected the Np 7p orbitals due to their relatively high energy with respect to the 5f, 6d, and 7s orbitals,30 which makes them less important in low-energy excitations. Accordingly, previous studies of the electronic spectra of NpO2+ and NpO22+ did not find any significant excitations to Np 7p orbitals.19,25,29

As MOLCAS can handle only Abelian point groups, the point groups of the target molecules (Drah and Crav) could not be applied. In order to be consistent for the mono- and dioxides, we used the C2v approach in our calculations. This might lead to symmetry breaking effects resulting in some cases to spurious orbital mixing. After investigation of the trial orbitals, the CLEAnup keyword of MOLCAS was applied to minimize as much as possible this shortcoming.

The ground state molecular geometries of the title Np oxides reported in Ref. 12 were applied in the present

study. The vertical excited electronic states were explored by multiconfigurational state-averaged calculations using up to 30 roots for a given spin multiplicity and symmetry. In addition to the ground-state spin multiplicity (4, 3, 2, 6, 5, and 4 for NpO2, NpO2+, NpO22+, NpO, NpO+, and NpO2+, respectively), generally the two neighbouring (lower and higher) ones were considered too. For details, see the supplementary material.76

All electron basis sets of atomic natural orbital type, developed for relativistic calculations (ANO-RCC) with the Douglas-Kroll-Hess Hamiltonian46,47 were used for all the atoms. For neptunium, a primitive set of 26s23p17d13f5g3h basis functions was contracted to 9s8p6d5f2g1h,48 whereas for O, a primitive set of 14s9p4d3f2g functions was contracted to 4s3p2d1f49 achieving TZP quality. The Douglas-Kroll-Hess Hamiltonian was used in the CASSCF calculations in order to take into account scalar relativistic effects.

Spin-orbit (SO) effects were taken into account by using the complete active space state interaction (CASSI) method,50 which allows CASSCF wave functions for different electronic states to interact under the influence of a spinorbit Hamiltonian. Dynamic electron correlation is taken into account using the CASPT2 energies as spin-orbit free (SF) energies in the spin-orbit Hamiltonian (SO-CASPT2). The above described multireference methods and the ANO-RCC basis set were successfully applied in a number of studies on actinide-containing systems.12,13,51-70

The electronic spectra have been evaluated from the Einstein coefficient values (being directly related to the spectral intensities) computed at the SO-CASPT2 level.71 The spectra were evaluated utilizing the relative populations of all computed SO states obtained by the Boltzmann equation. We note that our computed electronic transitions model the (unperturbed) gas-phase electronic spectra. When, however, electronic spectra are recorded in condensed phases, considerable environmental effects can appear. These effects can introduce drastic changes in the spectral intensities, while the transition energies suffer less from them.

III. RESULTS AND DISCUSSION A. NpO2, NpO2+, and NpO22+

Literature information on these molecules involves the ground electronic states of all three species and the electronic spectra of the NpO2+ and NpO22+ ions.12,19,23-26,29-35,38,39 We computed the excited electronic states up to ca. 35 000 cm-1 using the SO-CASPT2 method. As these states were obtained on the ground state geometries, the computations provided us the vertical excitation energies. On the basis of the excited states and the computed Einstein coefficients, we could model the gas-phase electronic spectra. The spectra were calculated at two temperatures, 298 K (to be compared with reported solution and solid-phase spectra at room temperature) and 3000 K (modeling high-temperature gas-phase studies). The calculated electronic spectra are presented in Figure 1. In Tables I-III, the most intense transitions for NpO2, NpO2+, and NpO22+, respectively, are compiled and characterized. The characterization is based on the major population differences

3000 K

5000 10000 15000 20000 25000 30000 35000

AE (cm ')

15000 20000 AE (cm1)

25000 30000 35000

0 5000 10000 15000 20000 25000 30000 35000 AE (cm ')

0 5000 10000 15000 20000 25000 30000 35000 AE (cm1)

0 5000 10000 15000 20000 25000 30000 35000 AE (cm1]

3000 K

0 5000 10000 15000 20000 25000 30000 35000 AE (cm1)

FIG. 1. Simulated gas-phase absorption electronic spectra of NpO2, NpO2+, and NpO22+.

between the donor and acceptor SO states. The full lists of the obtained SF and SO states are given in the supplementary material.76

In the spectra at 298 K, only excitations from the ground states appear with considerable intensity. Due to the population of low-lying excited states at high temperatures, the spectra simulated for 3000 K contain several additional lines compared to those at 298 K.

In the computed spectrum of NpO2 (Figure 1), the two major peaks at 15 101 and 27 398 cm-1 correspond to transitions 7s ^ 5fn and 5f ^ 6d§, respectively. At 3000 K, two another major bands join them at 23 752 and 30 024 cm-1 corresponding to transitions from the first excited state. We note that the main difference between the ground and first excited states of NpO2 is the inverted spin of the 7s electron. Thus, the former band represents a 7s^ ^ 5f<^ transition

Intensity

Energy 298 K 3000 K Einstein Fromb Tob Main orbitals involved

8175 6 6 376108 1(3.5g) 11(4.5u) 7s ^ 5fn

11632 6 565 672 2(4.5g) 24(5.5u) 7sß ^ 5fn

13 279 7 727 434 2(4.5g) 28(3.5u) 7sß ^ 5fn

13 861 10 1 008 360 2(4.5g) 29(3.5u) 7sß ^ 5fn

15101 20 20 1 325 410 1(3.5g) 31(2.5u) 7s ^ 5fn

16235 5 5 332 063 1(3.5g) 35(2.5u) 7s ^ 5fsß

17 725 5 5 302 602 1(3.5g) 40(2.5u) 7s ^ 5fs

19182 3 2 760 353 5(4.5g) 75(5.5u) 7s ^ 5f^ß

23 753 1 57 5 838 593 2(4.5g) 75(5.5u) 7sß ^ 5f+P

27 398 100 100 6744 513 1(3.5g) 84(3.5u) 5fs ^ 6d8

30024 31 3196 690 2(4.5g) 97(4.5u) 7sß5fs ^ 7s6ds

31 800 8 809 030 2(4.5g) 103(5.5u) 7sß5f^ ^ 5ff

aThe energies of transitions are given in cm-1, the relative intensities in % (using a threshold of 1%), and the Einstein coefficients in a.u.

bThe spin-orbit states defining the transitions; ^ values with parity in parentheses; for other details of the states, see the supplementary material STables 1 and 2.76

TABLE II. Significant computed electronic transitions (cm 1) of NpO2+.a

Intensity

Energy 298 K 3000 K Einstein Fromb Tob Main orbitals involved

15119 34 247 175 2(0g) 13(0g) 5f0, 2p0 ^ 5fs, 5f^

16011 57 57 90713 1(4g) 11(4g) ^ 5fs, 5fn, 2pn

19510 100 100 79499 1(4g) 15(3g) 5fs ^ 5fn, 2pn

23 877 58 207 982 2(0g) 26(1g) 5fs ^ 5fn, 2pn

24911 18 18 14573 1(4g) 24(3g) 5f^ ^ 5fn, 2pn

25 400 20 20 31 960 1(4g) 25(4g) 5fs ^ 5fn, 2pn

29 333 38 38 60100 1(4g) 33(4g) 5f^ ^ 5fn, 2pn

29 822 41 41 32622 1(4g) 35(4g) 5fs ^ 5fn, 2pn

34697 35 125 831 2(0g) 46(1g) 5fs ^ 5fnß,2pn

aThe energies of transitions are given in cm-1, the relative intensities in % (using a threshold of 1%), and the Einstein coefficients in a.u.

bThe spin-orbit states defining the transitions; ^ values with parity in parentheses; for other details of the states, see the supplementary material STables 3 and 4.76

TABLE III. Significant computed electronic transitions (cm 1) of NpO22+.a

Intensitya

Energy 298 K 3000 K Einstein Fromb Tob Main orbitals involvedc

13 806 4 2 729 959 6(4.5u) 20(4.5u) 5fs ^ 5f8ß,5f0,2p0

14642 54 7 372243 5(3.5u) 17(3.5u) 5f0 ^ 2p0

15 892 9 6 490 881 7(0.5u) 24(0.5u) 5f0, 2p0 ^ 2pn, 5fn

16018 4 6 892 856 8(1.5u) 31(1.5u) 5f0, 2p0 ^ 2pn, 5fn

16688 6 3 915 076 6(4.5u) 27(3.5u) 5f0 ^ 2p0

28 747 100 100 159422 1(2.5u) 25(3.5u) 5f0, 2p0, 5f^ ^ 5fs

29 302 10 10 16239 1(2.5u) 27(3.5u) 5f0, 2p0 ^ 5fs

30288 44 44 69 948 1(2.5u) 28(3.5u) 5f0,2p0, ^ 5f8,5f8ß

32959 26 26 42144 1(2.5u) 34(2.5u) 5f0, 2p0 ^ 5f^

aThe energies of transitions are given in cm-1, the relative intensities in % (using a threshold of 1%), and the Einstein coefficients in a.u.

bThe spin-orbit states defining the transitions; ^ values with parity in parentheses; for other details of the states, see the supplementary material STables 5 and 6.76 cP means the inverted spin.

(where p means the inverted spin), while the second, higher-energy, band corresponds to the double transition 7sp5f§ ^ 7s6d§. Other significant, but less intense transitions result in 7s ^ 5fn (at 8175 cm-1), some 7sp ^ 5fn and 7s ^ 5f§/^ ones below and above 15000 cm-1, respectively (cf. Table I). Double transitions appear characteristically above 30 000 cm-1.

In the ground and the studied excited states of NpO2+, the Np 7s orbital is not populated and the doubly occupied 5f0 seems to be very stable; thus, the electronic transitions correspond essentially to population transfer mostly from orbitals having major 5f and 5f^ contributions to ones having major 5fn contributions (cf. Table II). These originally electric dipole forbidden f ^ f transitions may become possible because the affected states contain minor contributions from 2p orbitals of oxygen.

The room-temperature computed spectrum of NpO2+ shows four intense transitions (Figure 1), those at 16 011 cm-1 f ^ 5f8, 5fn, 2pn), 19 510 cm-1 (5f ^ 5f„, 2p„), 29 333 cm-1 f ^ 5fn, 2pn), and 29 822 cm-1 (5f ^ 5f„, 2p„). Due to the notable population of the low-lying excited states at 3000 K, the latter spectrum contains several intense transitions between 15 000 and 35 000 cm-1. The main ones correspond to excitations from the ground and the first two excited states. Transitions accompanied with spin inversion appear at very high energy (e.g., the one at 34 697 cm-1).

We note that the Einstein coefficients (and accordingly the expected absolute intensities in the spectrum) of NpO2+ are generally smaller by one order of magnitude than those of neutral NpO2. This is due to the parity character of the states involved in the transitions. In the case of NpO2, all the donor states in Table I have gerade (g) and the acceptor states ungerade (u) parity satisfying the Laporte rule.72 In contrast, in NpO2+, all the states have a dominant gerade character. According to the Laporte selection rule, electronic transitions that conserve parity are forbidden. In the case of NpO2+, the minor mixing of ungerade 2p components in the SO states makes electronic transitions though possible, but with low probability. The case is similar for NpO22+, where all the states have a dominant ungerade character (cf. Table III).

Due to the decreased number of non-bonded valence electrons in NpO22+, its electronic spectra (Figure 1) are poor in signals. There are three intense transitions at 298 K, while an additional one at 3000 K. Several lower-energy transitions (below 20 000 cm-1) involve the excitation of a 5fa electron to an anti-bonding (2p,5f) orbital. The higher-energy transitions involve excitations of (mostly) a 5fa electron to another 5f orbital, but some double excitations appear also here.

Having reliable gas-phase electronic transition energies of the neutral and ionic NpO2 species, it can be interesting to see how they compare to previous experimental and theoretical investigations of the condensed-phase spectra of NpO2+ and NpO22+.16-20,23-25,30-35 Due to the environment influencing considerably the spectral intensities in solutions and solids, our Einstein coefficients computed for the isolated molecules may not be suited for comparison. In contrast, the transition energies suffer less from the environment. Therefore, our assessments are done on the basis of the computed SO-CASPT2 energies and character (Q value) of the excited states.

Selected experimental17,19 and theoretical results29,30,32 of NpO2+ are compiled in Table IV. The most recent study by Danilo et al.32 used SO-CASPT2 but with a considerably smaller active space (2 electrons in 6 orbitals) than ours. Yet, the agreement with our present SO-CASPT2 data is acceptable, the energy ordering of the excited states is the same in almost all cases, while the energy difference is mostly below 1000 cm-1. The agreement is better with the SO-MRCI results from that study (obtained by the small 2 electrons in 4 orbitals active space, corrected for size-extensivity according to Davidson73), though the latter results lack several high energy states.

Danilo et al.32 studied also the electronic states of hydrated NpO2+ modelled by the NpO2(H2O)5+ structure using SO-CASPT2 in conjunction with the smaller (2,4) active space. They showed that the H2O ligand field around NpO2+ resulted generally in minor changes in the excitation energies, while the average deviation between the calculated and experimental excitation energies was 1234 cm-1. The main advantage of this solvation model is, however, the intensity information. The ligands attached to NpO2+ can promote mixing of 5f^ and 6d§ orbitals, which can increase considerably certain transition probabilities. The computed oscillator strengths in Ref. 32 agree well with the experimental ones determined from the perchloric acid solution19 facilitating a reliable assignment.

As can be seen from Table IV, the Einstein coefficients of the NpO2+ molecule from our calculations are very different from the oscillator strength values of the NpO2(H2O)5+ model, confirming our above concern about the suitability of these intensity data of naked NpO2+ for the interpretation of condensed-phase spectra. On the other hand, the average deviation between the NpO2+ calculated and the experimental excitation energies is 1108 cm-1, slightly better than for those of the NpO2(H2O)5+ model32 obtained by the smaller active space. Particularly, our low-energy data are in good agreement with experiment19 (cf. Table IV).

The latest (our present and those of Danilo et al.32) computed data support generally the assignment proposed in the earlier IHFSCC study by Infante et al.30 Our calculations support also the findings of Infante et al.30 that the 5fn orbital appears only in higher-lying excited states; hence, transitions lower in energy than 19 000 cm-1 are unlikely to such orbitals.

Selected experimental20,23,24 and theoretical29-31,33,34 results of NpO22+ are compared in Table V. In the experimental spectra, five bands (two in solution20 and five in the Cs2UO2Cl4 crystal23) have been identified as 5f-5f electronic transitions from the ground state to low-lying excited states. The assignments of the charge transfer bands between 13 000 and 20 000 cm-1 have been done in terms of symmetry on the basis of crystal and Zeeman effect and magnetic circular dichroism measurements on the Cs2UO2(NO3)3 crystal and semi-empirical calculations.24

The theoretical studies compared in Table V include computations on the naked NpO22+ ion and on the NpO2Cl42-complex. The whole spectrum range covered by the available experimental data has been investigated in three studies: in the early SO-CI one by Matsika and Pitzer,29 in the recent SO-RASPT2 study of Su et al.,33 and in the present SO-CASPT2

TABLE IV. Computed vertical excitation energies (cm ') of NpO2+ compared with the spectra in aqueous perchloric acid solutions.

NpO2(H2O)5+

SO-CIa SO-MRCIb IHFSCCc SO-CASPT2b SO-CASPT2d SO-CASPT2b Expt.e Expt.f

State E E E E E Einstein E Oscillatorg State E State E Oscillato

4g 0 0 0 0 0 0 0 0

0g+ 3 366 3143 2527 2364 3 169 0 2326 0

lg 4938 5 283 4102 3 722 4949 0 4243 15.5

5g 4721 6342 5 379 6528 6144 155 6585 0 3H5 6173

0g- 9708 10734 8 628 9177 9 275 4 10101 0.4 3n0 8 953 3n0g 8 953 10

lg 9076 10 469 8 929 9 687 9755 0 9719 156.0 3^1 9 146 3£1g 9116 136

0g+ 9537 11 829 9 378 10068 10382 0 11 094 0.2 3n0 9 780 3n0g 9 777 40

2g 11187 12392 10056 10697 10992 0 11 740 953.8 3n2 10 208 3n2g 10 202 1200

6g 8 867 11 574 9 690 11 816 11253 1 074 12135 0 3r3 13 020 3®2g 12995

0g+ 14415 16 628 14105 15 299 15 869 0 16231 0 £0 13 824

4g 15 249 18 394 14422 14533 16011 90 713 16373 0 1r4 14 577 1n1g 14558

lg 16156 18321 15 031 16678 17 051 0 17 832 65.1 3£1 16 220 3A2g 16221 181

0g+ 19647 19170 16551 18017 18 289 0 18918 0.5 A2 16 906

3g 24 834 19735 19 883 19510 79 499 3A1 18116

5g 22031 22 530 19761 21 496 22 521 1 145 21 930 0 2I6 21 008 3n0g 21 004

lg 21 672 22 045 18 992 21 041 20911 0 22 408 280 3n0 22 600 3£1g 23 245 532

6g 23 327 22 649 20 035 23 813 22 923 0 25 728 0

2g 23 649 23187 21 852 22 879 0

2g 23 322 23 089 23 433 0

4g 26 592 25119 25 778 25 400 31 960

lg 23 079 25 436 25 826 27 046 0

lg 27 494 27 916 0

3g 28 068 29 087 0

3g 28 416 30 846 116

0g+ 32 809 29 649 32248 0 31 032 0.2

aSO-CI computations of Matsika and Pitzer.29

bFrom Ref. 32 using (2,4), (2,6), and (2,4) active spaces in the SO-MRCI and the two SO-CASPT2 calculations, respectively.

co o cn

cIHFSCC computations of Infante et al.30 dPresent computations including the Einstein coefficients (

i.u.) for excitations from the ground state.

e 1M perchloric acid solution measured by Waggener interpretation revised by Eisenstein and Pryce. Additional experimental peaks: 16 100 cm" -1 (3®2), 19 360 (n1), 21 700 (3A3). f0.1M perchloric acid solution measured by Matsika et al.119 gOscillator strength x10-7 obtained with the (2,4) active space. hOscillator strength x10-7.

co o cn

TABLE V. Computed excitation energies (cm of NpO22+ and NpO2CLi2 compared with experimental data.

NpO2 computed

NpO2Cl42 computed

SO- SO- SO-

SO-CI IHFSCC RASPT2 SO-CASPT2 Experimental IHFSCC RASPT2 CASPT

State Ea Eb Ec d T e Ee Ef Einsteinf Eg Eh Ei Ec d Te Ee

2.5u 0 0 0 0 0 0 0 0 0 0 0 0

1.5u 447 3 544 3 221 3 503 3011 2 426 9 1 000 1 156 1 055 944

3.5u 6 565 7 227 7 225 6 107 8092 8 158 49 6760 6 459.0 6 880 7 738 5 767 7759

2.5u 5 515 8 929 8 565 7 798 9192 8 241 537 8180 9 420.2 7 990 9137 6658 8600

3.5u 12 622 9 498 9 293 105 13918.1 13 264.9 11127

0.5u 15 668 13 816 12813 0 16 072.5 15 406.4 14375

4.5u 15 418 12 490 12613 0 16092.6 15 683 14122

1.5u 16 664 15 015 14 663 2 17 967.7 16799.8 15 330

1.5u 25 844j 29441 19 337 19759 53 17 843.6 17 241.4 20 857 17129

1.5u 21 580j 17 282 21 093 4 19510.2 19375.2 18 774

1.5u 28 909 33 856 34947 19318 24913 0 20816.3 20 080.8 26305 20 134

aVertical excitation energies from SO-CI computations of Matsika and Pitzer.29

bVertical excitation energies from IHFSCC computations of Infante et al.3 c Vertical excitation energies from IHFSCC computations of Gomes et al?1 For NpO2Cl42-, they used an embedding potential obtained from DFT calculations. dAdiabatic excitation energies from SO-RASPT2 calculations using restriction schemes of S(2,4) and S(2/2) for NpO22+ and NpO2Cl42-, respectively.33 eVertical excitation energies from SO-CASPT2 calculations of Gendron et al.34 using a (7,10) active space.

f Vertical excitation energies from the present computations and the Einstein coefficients (a.u.) for excitations from the ground state. gFrom aqueous HCl solution.20

hSolid state spectrum, NpO22+ doped into Cs2UO2(NO3)3 crystal.23,24 ' Solid state spectrum, NpO22+ doped into Cs2UO2Cl4 crystal.23,24

J These energies were assigned to acceptor states with 0.5u character in Ref. 29.

calculations by us. From these studies, Su et al. calculated both NpO22+ and NpO2Cl42-, providing information on the effect of chloride ligands on the transition energies. Another advantage of this study (making it the best model for the doping experiment of NpO22+ into Cs2UO2Cl4 crystal23,24) is the evaluation of the adiabatic transition energies. As can be expected, this study achieved the best average deviation (947 cm-1) from experiment. Those of the SO-CI and SO-CASPT2 energies of the naked NpO22+ are around 2400 cm-1. Considering the 0-10 000-cm-1 region of the spectrum, the best agreement with the experimental data of Cs2UO2Cl4 (515 cm-1) has been obtained by Gendron et al., who computed only the three lowest excitations of this molecule using SO-CASPT2.34 We note that to the better agreement in the latter limited study, the following character of CASPT2 may also contribute: the energies of excited states become less accurate with increasing number of states considered in the state-averaged calculations.

The assignments of the experimental transitions to the acceptor (excited) states are generally in agreement with our and in the listed literature studies.24,29-31,33,34 Instead of the Q = 0.5u excited states suggested for the experimental transitions at 19 510 and 17 844 cm-1 by Denning,23,24 our and the other recent computations predicted more suitable excited states with Q values of 1.5.

B. NpO, NpO+, and NpO2+

The electronic ground states of these species were reported recently by Infante et al.12 from SO-CASPT2 calculations. The ground and low-lying excited states are strongly mixed. We observed in the composition of the ground

states of the three molecules determined in our present study some minor differences from those in Ref. 12 which, however, do not affect the Q quantum number of these states. We attribute these differences to the inclusion of more states in the CASSCF and CASSI calculations, which result in more extensive mixing than occurred between the few ones (three states for each species) in Ref. 12.

The details of the electronic ground and excited states are given in the supplementary material.76 With exception of the ground states, nothing is known about these monoxides; therefore, we briefly discuss here a few important characteristics of their electronic structure.

The major components of the SO ground state of NpO arrive from the SO coupling of the 6® and 6A low-energy SF excited states (ca. 1000 cm-1 above the SF ground state). Accordingly, the major Np orbital populations include the 7s and the four 5f (a, n, ô, orbitals. These are the major orbitals in the low-lying excites states too, only with somewhat different contributions. The 6dô orbitals appear with notable (but still minor) contribution above 13 000 cm-1. Their importance is increased in the higher excited states. These characteristics can be compared with those of the isoelectronic PuO+.12,74 The latter species has a simpler electronic structure than NpO: it has one major (79%) contribution with no 7s occupation and also the 5fa orbital has only a minor role in its electronic ground state. This character is preserved in the reported few low-energy (0-3000 cm-1) excited states of

PuO+.74

The SO ground state of NpO+ consists mainly of the SO-coupled 5® SF ground and 5A SF low-energy excited states. The major Np orbital populations include the four 5f (a, n, ô, orbitals. The 7s orbital, being important in

— 50-

10000 20000 AE (cm1)

0 5000 10000 15000 20000 25000 30000 35000 AE (cm1)

5000 10000 15000 20000 25000 30000 35000

AE (cm1)

3000 K

0 5000 10000 15000 20000 25000 30000 35000 AE (cm1)

0 5000 10000 15000 20000 25000 30000 35000 AE (cm1)

3000 K

0 5000 10000 15000 20000 25000 30000 35000 AE (cm ')

FIG. 2. Simulated gas-phase absorption electronic spectra of NpO, NpO+, and NpO .

the neutral NpO, became a minor contributor in the ion and gains importance in some high-energy excited states only. The same refers to the 6d8 orbitals too. Compared to the characteristics of isoelectronic UO, the main difference is the monoconfigurational character and dominant 7s contribution in the uranium oxide, in the ground state of which the 5fa orbital plays no role.12,75 However, the 7s and 5fa orbitals

appear as minor contributions in some high-energy excited states of UO.75 The case is similar in the isoelectronic PuO2+ with a major (70%) SF state of 5f§2, 5f<^, 5f^ character. In the ground and excited states of the latter species, 5fa has been found as a minor contributor.12,74

The SO ground state of NpO2+ consists mainly of the 4I SF ground state with major Np 5fn\ 5f§\ 5f<^ populations. The 5fa

Intensity

Energy 298 K 3000 K Einstein Fromb Tob Main orbitals involved0

11160 14 12 23 779 1(1.5) 55(2.5) 7s, 5f§ ^ 5fn

13 992 12 37 571 3(2.5) 111(2.5) 5f0, 5f§ ^ 6d8

14 880 29 23 47 966 1(1.5) 110(2.5) 5f0, 7s, 5fg ^ 6dg

14997 32 26 53 342 1(1.5) 112(2.5) 7s, 5f0, 5f^ ^ 6dg

15122 29 23 48165 1(1.5) 114(2.5) 5fg, 7s ^ 6dg

15 818 28 86402 2(2.5) 146(2.5) 7s, 5fg, 5f^ ^ 6dg, 5fg'

16093 24 74 602 2(2.5) 150(2.5) 5fg, 7s, 5fn ^ 5f^, 5f§', 6d8

16227 27 85 945 2(2.5) 153(2.5) 7s, 5f§ ^ 6d§

17 093 34 27 56066 1(1.5) 153(2.5) 7s, 5f^ ^ 6dg

17159 33 27 55112 1(1.5) 154(2.5) 5fg, 5f0 ^ 6d8, 5f^

18 800 36 29 60 756 1(1.5) 187(2.5) 5fg, 7s, 5f0 ^ 6dg, 5fg'

21 803 34 28 57 235 1(1.5) 259(2.5) 5fg, 7s ^ 6dg, 5fg'

23 388 27 22 45143 1(1.5) 303(2.5) 5fg, 5f^, 7s ^ 6dg, 5fg'

23 417 35 28 58183 1(1.5) 304(2.5) 5f§, 5f^, 7s ^ 6d§,5f§'

23 957 23 125 282 4(2.5) 365(3.5) 5fg, 7s, 5f0 ^ 6dg, 5fg'

24 340 45 284602 5(2.5) 380(2.5) 7s, 5fn ^ 6d8, 5f0, 5fn'

25 270 27 22 45 575 1(1.5) 350(2.5) 7s, 5f^, 5f§ ^ 6d8, 5fn

25 284 30 97 020 3(2.5) 370(3.5) 5fg, 7s, 5f0 ^ 6dg, 5fg'

25 323 61 176305 2(2.5) 370(3.5) 5f^, 5fs, 7s ^ 6d8, 5fs', 5fn

25 827 44 129 411 2(2.5) 380(2.5) 7s, 5fg ^ 6dg, 5fg'

26 694 100 81 166641 1(1.5) 380(2.5) 7s, 5fs, 5fn ^ 6d8, 5fn'

26 798 27 145 262 4(2.5) 410(3.5) 7s, 5fg, 6d0 ^ 6dg, 5fg'

26 904 22 70 152 3(2.5) 397(2.5) 7s, 5f§ ^ 6d8, 5fn

27 193 34 104 810 2(2.5) 402(3.5) 7s, 5fs, 5f^ ^ 5f^', 5fs', 6dn, 6d8

27 258 27 84769 2(2.5) 403(2.5) 7s, 5fg, 5f^ ^ 6dg, 5fg'

27 658 42 132589 2(2.5) 407(2.5) 7s ^ 5fg, 6dg

27 910 36 29 59317 1(2.5) 401(2.5) 7s, 5fg, 5f^ ^ 5fg', 6dg

27 942 45 261 397 5(2.5) 424(3.5) 5f^ ^ 5f^', 7s

28 087 30 166228 4(2.5) 423(2.5) 5f§, 5f^, 5f0 ^ 5f§', 5f^', 6d8

28 525 97 78 161 934 1(1.5) 407(2.5) 7s, 5fg, 6d0 ^ 6dg, 5fg', 5fn

28 676 37 30 60 989 1(1.5) 409(2.5) 7s, 5fg ^ 6dg, 5fg'

28718 24 129 850 4(2.5) 427(2.5) 5f§, 5f^, 5f0 ^ 5f§', 5f^', 6d8

29 255 22 69 159 2(2.5) 423(2.5) 5f^, 5f§ ^ 5f^', 5f§', 6d8

29 580 67 211 966 3(2.5) 425(2.5) 5f^,5f0 ^ 5f^', 7s, 5f§

29 847 38 120500 3(2.5) 427(2.5) 5f§, 5f^, 5f0 ^ 5f§', 5f^', 6d8

29 886 39 122254 2(2.5) 427(2.5) 5f^, 5fg ^ 5f^', 5fg'

30295 51 41 85144 1(1.5) 424(2.5) 5f^, 5f§ ^ 5f^', 5f§', 7s

30389 100 312750 2(2.5) 428(2.5) 5f§, 5f^, 7s ^ 5f§', 5f^'

30485 47 38 78 062 1(1.5) 425(2.5) 5f^ ^ 5f^', 5f§, 7s

31 256 62 50 103 892 1(1.5) 428(2.5) 5fg, 7s, 5f^ ^ 5fs', 5f^', 6d8

aThe energies of transitions are given in cm-1, the relative intensities in % (using a threshold of 1%), and the Einstein coefficients in a.u.

bThe spin-orbit states defining the transitions; Q values in parentheses; for other details of the states, see the supplementary material STables 8 and 9.76

cThe symbols 5fn', 5fg', and 5f^' mean the orbitals with magnetic quantum number of the opposite sign to that of the donor one.

orbital gains importance in the first excited state having a 4H character, then in a few strongly mixed states above 8000 cm-1. The 6dn orbital appears as minor contributor in strongly mixed excited states above 10000 cm-1. The only isoelectronic actinide oxide with available literature data is UO+ with analogous ground and first excited states.75 However, the 5f0 orbital seems to have a larger role in the excited states of UO+ than found in NpO2+.

The calculated electronic spectra for 298 K and for 3000 K are presented in Figure 2. In Tables VI-VIII, the most intense transitions for NpO, NpO+, and NpO2+, respectively, are compiled and characterized.

A brief comparison of the monoxide spectra and data with those of the dioxides reveals considerably more excited states and electronic transitions within the covered energy window of 0-35 000 cm-1. The reason is the larger number of unbound Np valence electrons in the monoxides resulting in an increased number of possible electron configurations. Another characteristic of the monoxides is, as compared to the above discussed dioxides, the considerably more extensive mixing in the electronic states. The majority of the excited states consist of several electron configurations with a low contribution being impossible to pick a single electron configuration as dominant contribution. Thus, instead of excitations of well-

Intensity

Energy 298 K 3000 K Einstein Fromb Tob Main orbitals involved

14905 15 349 513 4(3) 78(3) 5fn ^ 7s

17 363 24 402 772 3(2) 102(2) 5fg, 5f0 ^ 7s, 6dg

17 950 5 37 538 741 2(1) 109(1) 5f0, 5fs ^ 6d8, 5fn

18 063 25 418 070 3(2) 111(2) 5f0, ^ 6d8

19744 22 492 002 4(3) 147(3) 5f^, 5fg, 5f0 ^ 6dg, 7s

19797 15 97 1402 516 2(1) 134(1) 5f^, 5fg, 5f0 ^ 6dg, 7s

19916 4 25 364 050 2(1) 136(1) 5f^, 5f0, 5f§ ^ 5fn, 7s, 6d8

20 373 100 91 1 053 352 1(0) 136(1) 5fs, 5f0, 5f^ ^ 5fn, 7s, 6d8

21 822 6 43 626 053 2(1) 162(2) 5f^, 5fs, 5f0 ^ 5fn, 7s, 6d8

22 225 67 61 709 038 1(0) 161(1) 5fg, 5f0, 5f^ ^ 6d8, 7s, 5fn

22 334 9 59 848 473 2(1) 170(2) 5f^, 5f0, 5fs ^ 6d8, 7s, 5fn

24 764 27 450 982 3(2) 244(2) 5f^, 5fg ^ 6dg, 7s

27 322 47 22 499 279 1(0) 283(1) 5fg, 5f0 ^ 5fn, 6d8

28 499 23 10 238 825 1(0) 306(1) 5fg, 5f^ ^ 7s, 5fn, 6dg

28 738 100 3106 203 5(4) 376(3) 5fg, 5f^ ^ 6d8

28 744 52 1 623 235 5(4) 377(4) 5fg, 5f^, 5fn ^ 6d8

29 877 23 708 032 5(4) 410(3) 5f^, 5fs ^ 6d8

33 447 24 395158 3(2) 447(2) 5f^, 5fg ^ 6dg, 7s

33519 44 746 053 3(2) 448(1) 5f^, 5fs ^ 6d8, 5f0

34061 82 1846 990 4(3) 463(2) 5f^, 5fs ^ 6d8, 5f0, 6d0

34680 21 707 611 3(2) 463(2) 5f^, 5fs ^ 6d8, 5f0

34786 11 10 114917 1(0) 452(1) 5f^, 5fs ^ 7s, 5fn

aThe energies of transitions are given in cm-1, the relative intensities in % (using a threshold of 1%), and the Einstein coefficients in a.u.

bThe spin-orbit states defining the transitions; Q values in parentheses; for other details of the states, see the supplementary material STables 10 and 11.76

defined electrons to another orbital, the transitions in these species are characterized by transfer of small electron density fractions.

From the three species, the electronic spectra of NpO (Figure 2) are the most crowded, even at room temperature. There are four and five (at 298 K and 3000 K, respectively) very intense transitions between 25 000 and 33 000 cm-1. All the listed significant transitions at 298 K occur from the ground state. The most intense excitation (at 26 694 cm-1) corresponds to a population transfer from the 7s, 5f§, and 5fn

orbitals to 6ds and 5fn', the latter meaning the 5fn orbital with opposite-sign magnetic quantum number with respect to the donor 5fn.

At 3000 K, transitions from low-lying excited states (up to the fourth excited state at 2354 cm-1) appear with considerable intensity. The most intense band in the covered range is the one computed at 30 398 cm-1 representing a major 5f§, 5f^, 7s ^ 5f§', 5f</ charge transfer. The majority of the transitions involve the Np 7s orbital as donor and the 6ds orbital as acceptor. From the 5f orbitals, 5f§ is the

TABLE VIII. Selected significant computed electronic transitions (cm 1) of NpO2+.a

Intensity

Energy 298 K 3000 K Einstein Fromb Tob Main orbitals involved

8719 1 1 6184 1(4.5) 9(3.5) 5fn ^ 5fs

9153 2 68 537 4(1.5) 23(1.5) 5fn, 5f§ ^ 5f0, 5f^

11 193 2 185 586 6(0.5) 37(0.5) 5fn ^ 5f^

11211 2 2 9 853 1(4.5) 16(3.5) 5fn, 5f^ ^ 5f0, 5fs, 6d0

19733 4 4 20 844 1(4.5) 46(5.5) 5fn ^ 5f0

22 285 2 37 469 2(3.5) 65(2.5) 5f0, 5fs, 6d0 ^ 5f^, 5fn

23 904 2 2 9 500 1(4.5) 61(5.5) 5fs ^ 5fn

27 205 10 514373 5(5.5) 112(6.5) 5fn ^ 6d8

27 273 100 100 510090 1(4.5) 80(5.5) 5fn ^ 6d8

27 672 4 77 239 2(3.5) 104(2.5) 5f0, 6d0 ^ 5fn

28 553 2 36792 2(3.5) 109(3.5) 5f0, 6d0 ^ 5fn

aThe energies of transitions are given in cm-1, the relative intensities in % (using a threshold of 1%), and the Einstein coefficients in a.u.

bThe spin-orbit states defining the transitions; Q values in parentheses; for other details of the states, see the supplementary material STables 12 and 13.76

most frequently involved in these processes. In the very high-energy transitions (above 28 700 cm-1), the donor role of 7s is decreased, and it appears instead as acceptor in a few excitations.

The electronic spectra of NpO+ are considerably less crowded than those of NpO, but still include plenty of weak transition lines. Similarly to NpO, the most intense transitions appear at and above 20000 cm-1. Most excitations in the 298-K spectrum occur from the ground electronic state, but because of the notable population of the close-lying first excited state (at 457 cm-1, cf. Table VII), transitions from this state appear with some intensity (up to 15%) too. The most intense line (at 20373 cm-1) corresponds to a 5f§, 5f0, 5f^ ^ 5fn, 7s, 6ds population transfer from the ground electronic state. Though this transition is very intense in the spectrum at 3000 K too, the main one there (at 28 738 cm-1) occurs from the fourth excited state with somewhat different population transfer characteristics (5f§, 5f^ ^ 6d§). Regarding the role of Np atomic orbitals in the transitions, the main difference compared to neutral NpO is the lack of 7s orbital from the donor (ground and low-lying excited) states. Instead, 7s appears as an important acceptor in ca. half of the acceptor states. The most frequent donor orbitals are 5f§ and 5f^, while the most frequent acceptor orbital is 6d§, accompanied in several cases by 7s and 5fn.

In contrary to the previous two monoxide species, the computed electronic spectra of the NpO2+ ion at both 298 K and 3000 K are poor of intense transitions. They include a single intense line only (at 27 273 cm-1, cf. Figure 2 and Table VIII) attributed to a 5fn ^ 6d§ population transfer. The Einstein coefficient of this transition is comparable to the intense ones in the spectra of NpO but is smaller by ca. one order of magnitude than those of NpO+. Differences with respect to NpO and NpO+ appear in the Np orbitals important in the transitions: In NpO2+, the 5fn orbital does not play an important role; the most frequent donor orbital is 5fn. The acceptor ones cover 6d0,6d§, and all the four types of 5f. As 7s is poorly populated in NpO2+, it does not have any important role in its transitions.

IV. CONCLUSIONS

We studied the low-lying excited states and corresponding gas-phase electronic spectra of the neutral and ionic NpO and NpO2 molecules. Our computations at the SO-CASPT2 level of theory elucidated the parameters and characters of electronic transitions. Spectral intensities were estimated on the basis of the computed Einstein coefficients for room temperature (298 K) and for 3000 K, being more relevant for the gas-phase experiments. From the six studied species, the dioxides (NpO2, NpO2+, NpO22+) and NpO2+ have quite simple electronic spectra possessing only a few intense peaks. On the other hand, the spectra of NpO and NpO+ are very dense. The electronic states of these two species are strongly mixed and in several cases only small population transfers between various Np orbitals could be recognized. The computed vertical transition energies show good agreement with the previous results on NpO2+ and NpO22+.

ACKNOWLEDGMENTS

The 7th Framework Programme of the European Commission under Grant No. 323300 (TALISMAN), the Hungarian Scientific Research Foundation (OTKA No. 75972), is acknowledged for financial support.

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