Electronic Transitions and Vibronic Coupling in Neptunyl Compounds

Article pubs.acs.org/JPCA

Electronic Transitions and Vibronic Coupling in Neptunyl Compounds Guokui Liu,*,† Shuao Wang,‡ Thomas E. Albrecht-Schmitt,‡ and Marianne P. Wilkerson§ †

Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, Illinois 60439, United States Department of Chemistry and Biochemistry and Department of Civil Engineering and Geological Sciences, University of Notre Dame, Notre Dame, Indiana 46556, United States § Chemistry Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States ‡

ABSTRACT: The low-lying electronic transitions of the neptunyl (NpO22+) ion are characterized as either charge transfer (CT) or intra- 5f. Comparison of these classes of electronic transitions reveals significantly different photophysical properties, especially in vibronic coupling. An empirical model developed for analyses of uranyl CT vibronic transitions is used here to simulate the absorption (excitation) spectra of neptunyl in two compounds of different chemical compositions and structural symmetries. Analyses reveal that CT vibronic coupling in neptunyl has the same characteristics as that in typical uranyl analogues. The primary profile of the CT spectra is similar for neptunyl respectively with respect to chloride- and oxide-neptunium bonding interactions. On the other hand, vibronic coupling to the CT transitions is significantly different from that of f-f transitions, even within a given neptunyl compound. Electronic energy levels, vibronic coupling strength, and frequencies of various vibration modes were evaluated for transitions to the excited states of different origins in the region from 8000 cm−1 to 21000 cm−1 for two neptunyl compounds.

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INTRODUCTION Optical spectra that reveal the electronic transitions of actinyl ions in compounds have been extensively used as a probe of ion-ligand coordination and speciation.1−5 A unique characteristic in the actinyl spectra is charge transfer (CT) vibronic transitions that provide experimental information on actinyl electronic structure and its influence on chemical bonding.6−10 It was demonstrated that the absorption and photoluminescence spectra of uranyl (UO22+) in compounds and complexes can be simulated using an empirical model of vibronic coupling established on the basis of Huang−Rhys theory of ion-lattice interaction.11−13 These simulations provide a quantitative understanding of the CT vibronic transitions in uranyl compounds. For example, the energies of electronically excited states, vibrational frequencies of various local modes, and changes in the geometry of the uranyl complexes were evaluated for several electronic states and coordination geometries. Because these simulations involving the simplest of actinyl (U(VI)O22+) systems were successful, we attempted to apply these simulations to the isostructural f1 (Np(VI)O22+ actinyl system.10,14,15 The presence of a single electron in the 5f valence shell of NpO22+ opens up the opportunity to probe the properties of intra-5f transitions without introducing the complexity of a 5fn>1 electronic configuration. In contrast to the electronic configuration of uranyl, the configuration of the ground state of neptunyl comprises a singly occupied nonbonding 5f orbital. Here, electronic transitions of neptunyl in compounds have two different origins. One is © 2012 American Chemical Society

similar to those observed in uranyl compounds. Namely, CT transitions occur when an electron in a ligand centered orbital (valence state) is excited into a nonbonding 5f metal-based orbital.10,16,17 Transitions occur from the ground 5f1 state to the excited states of primarily the same 5f1 configuration but different spin and crystal-field states known as f-f transitions. Fluorescence emission rising from 5f-5f transitions of neptunyl in both single crystals and aqueous solutions has been experimentally demonstrated.18,19 In absorption and excitation spectra, the progressive vibronic bands of CT transitions overlap with the f-f transitions in the spectral region from 13,000 cm−1 up to above 20,000 cm−1.5,20,21 Denning and co-workers reported the absorption spectra of doped crystals of Cs2Np(U)O2Cl4 and CsNp(U)O2(NO3)3 at liquid helium temperature and assigned the zero-phonon lines (ZPLs) of electronic transitions and some of the vibronic bands for both ff and CT transitions.14,15 It was shown that the spectra of the two systems have similar structure, and the characteristics of the CT vibronic transitions have a strong resemblance to those of the isostructural uranyl in the same crystals. Similar results have been reported later by different groups.20,22 Vibronic coupling is significant in both f-f and CT transitions, but exhibit different characteristics in terms of mode frequencies and coupling strength. The complexity of the spectra because of a high Received: March 20, 2012 Revised: July 9, 2012 Published: July 11, 2012 8297

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Figure 1. Crystal structures of Na[(NpO2)B5O8(OH)F](H2O) and neptunyl cluster. Np atoms are shown in yellow, O atoms in red, B atoms in blue; F and Na atoms are shown in green and violet, respectively. OH and water molecules are not shown.21.

with Cs2NpO2Cl4, neptunyl in Na(NpO2)[B5O8(OH)F]H2O has a very different ligand geometry. The quantitative and comparative analysis of the CT and f-f vibronic transitions in these two systems has provided new information and advanced our understanding of ion-ligand interaction in actinyl complexes.

density of electronic states and progressions prevent unambiguous assignment. Ab initio calculations provide theoretical insights into analysis of energy level structures. Calculations provided by Pitzer and co-workers16,17 achieved an energy level structure scheme that included transitions between electronic states of both CT and f origins. Su et al.23 recently reported theoretical calculations of f-f and CT transitions in neptunyl ion and its complex of [NpO2Cl4]2−. The agreement between the calculated energies and the experimentally observed ones is improved except for those with energies above 20,000 cm−1. On the basis of the analyses of f-f and CT transitions of neptunyl in chlorides and nitrate crystals conducted by Denning et al.,14,15 a more systematic and quantitative analysis of neptunyl spectra of optical absorption, including both the f-f and CT vibronic transitions, is possible to provide essential information about neptunyl electronic and vibronic interactions in the excited states. Unlike the ligand sensitive CT transitions such as those observed in lanthanide compounds,24,25 the CT transitions in neptunyl compounds are more localized within the oxo ion predominantly from the O-2p orbitals to the Np-5f(6d) orbitals. They are less sensitive to ligand environments, and thus have energy levels not strongly dependent on ligands and sharp line width in both ZPLs and vibronic side bands. Therefore, the rich and well-resolved spectroscopic structures in the absorption (excitation) and fluorescence spectra of an actinyl compound enable more detailed characterization of CT vibronic interactions. The 5f orbitals of neptunyl are divided into nonbonding (fδ, fϕ) and bonding (fσ, fπ) orbitals.14,15 Vibronic coupling of the f-f transitions between nonbonding states are expected to be less significant than that involved in a bonding state. A quantitative analysis of CT and f-f transitions is expected to provide new insights into the fundamental properties of electronic interactions in actinyl compounds. In this paper, we report the spectroscopic properties of two neptunyl compounds, Cs2Np(U)O2Cl4 and Na(NpO 2)[B5O8(OH)F]H2O and demonstrate how a quantitative understanding of vibronic transitions is achieved via theoretical simulation of the experimental spectra. In Cs2Np(U)O2Cl4, Np(VI) and U(VI) occupy the same lattice site. The energy level structure and vibronic transitions in Cs2NpO2Cl4 were previously investigated by several groups.9,14,20 The present work extended the spectroscopic analysis to determine the systematic behaviors of CT and f-f transitions. In comparison

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STRUCTURES AND SPECTRA OF TWO NEPTUNYL COMPOUNDS Similar to most uranyl crystals, neptunyl in both compounds forms layered crystalline structures. The structure and vibration modes of Cs2NpO2Cl4 are identical to those of the well-studied Cs2UO2Cl4.10 In fact, the present data were obtained from a sample of Cs2Np(U)O2Cl4 in which Np(VI) and U(VI) have identical structure and geometry. Saturation of the neptunyl ion is completed by coordination of four chloride ligands in the equatorial plane. Including the intrinsic ONpO modes, named symmetric and asymmetric stretching modes ν1 and ν2 and bending mode ν3, there are 11 orthogonal vibration modes in the octahedral complex. Wilkerson et al.26 found that in the lattice of Cs2Np(U)O2Cl4 the actinyl bond length and symmetric stretching frequency changed from R(U−O) = 1.774(4) Å; ν1(U−O) = 832 cm−1 to R(Np−O) = 1.775(17) Å; ν1(Np−O) = 802 cm−1. The changes are consistent with the correlation between the frequency of the OAnO stretching mode and the bond length.10,27 The chloride ligand modes have frequencies separated into two groups, four Np−Cl bending modes and one O−Np−O rocking mode with frequencies expected around 120 cm−1, and three Np−Cl stretching modes with frequencies close to 250 cm−1. The structure of Na(NpO2)[B5O8(OH)F]H2O shown in Figure 1 was recently reported.21 Each Np atom is coordinated with eight oxygen atoms (NpO8) in a hexagonal bipyramidal geometry. The pyramids are based on axial neptunyl groups (NpO22+), and six oxygen atoms coordinate the NpO22+ groups in the equatorial plane. The bond lengths of Np−O in NpO22+ are about the same as that of U−O in UO22+ in the same complex, which range from 1.74 to 1.78 Å. Therefore, ionligand interaction and vibronic coupling in a neptunyl compound are expected to be similar to that of a uranyl compound. In comparing between Cs2NpO2Cl4 and Na(NpO2)[B5O8(OH)F]H2O, the major difference is that NpO22+ in Cs2NpO2Cl4 has four Cl− ligand ions in the equatorial plane, where in Na(NpO2)[B5O8(OH)F]H2O it has six O2− ligand ions. The differences in spectroscopic character8298

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Table 1. Energy Levels of 5f Electronic and CT States, Stretching Mode Frequency, and Vibronic Coupling Constant of Neptunyl (NpO22+) in Compounds configuration main term (D∞h)

|Jz| (D∞h)

Φu 2 Δu 2 Δu 2 Φu 4 Hu 4 − Σu 4 Hu 4 − Σu 2 Πu 4 Πu 4 Hu 4 Hu 4 Πu 2 Πu 4 Πu 4 Πu

5/2 3/2 5/2 7/2 7/2 1/2 9/2 3/2 1/2 1/2 11/2 13/2 3/2 3/2 1/2 5/2

2

f/f1 f/f2 f/f3 f/f4 σf2/CT1 σf2/CT2 σf2/CT3 σf2/CT4 f/f5 σf2/CT5 σf2/CT6 σf2/CT7 σf2/CT8 f/f6 σf2/CT9 σf2/CT10

NpO2Cl42− RASPT2/SO (Su et al.23)

Cs2NpO2Cl4 (Denning et al.14,15)

Cs2NpO2Cl4 (this work)

Ee/cm−1

v1/cm−1

E0/cm−1

v1/cm−1

0 1055 5767 6658 11127 14375 14122 15330

786 812 809 800 664 665 663 662

802

0

722 715 711 713

18774 17129 20134 20305

663 663 661 663

0 ∼1000 6880 7890 13265 15406 15683 16800 17241 19375 20700

13265 15366 15648 16780 17250 19345

723 713 709 715 790 708

0.9 1.05 1.05 1.10 0.55 1.05

20051

790

0.55

20372 20537

658 661

20081

686

E0/cm−1

v1/cm−1

S

Na[(NpO2)B5O8(OH)F] (H2O) (this work) E0/cm−1

v1/cm−1

S

760 720 720 720 720 760 720

0.3 0.9 0.9 0.9 0.9 0.5 0.9

0

8845 13128 15213 15463 17073 17510 18980

istics should be primarily due to the variation in the ligand environments and structural geometries. The site symmetry in Cs2Np(U)O2Cl4 is D2h which can be considered as a slightly distortion from D4h.8 The neptunyl site symmetry in Na(NpO2)[B5O8(OH)F]H2O is a descent of D3h. On the basis of the symmetry properties of crystal-field splitting, there are same number of crystal field doublets in the two systems, which are labeled by the same name in Table 1. The excitation spectrum shown in Figure 2 was obtained from a single crystal of Cs2Np(U)O2Cl4 at 75 K in which

Figure 3. Experimental and simulated absorption spectra of Na[(NpO2)B5O8(OH)F](H2O) at 295 K. The ZPLs of CT and f-f transitions are marked by the arrows.

excitation spectrum of Cs2Np(U)O2Cl4 (Figure 2) and the absorption spectrum of Na(NpO2)[B5O8(OH)F]H2O (Figure 3) show a correlation in energies and intensities. In the lower energy portion, there is a group of weaker progressing peaks between 13000 and 15000 cm−1 with origin at 13265 cm−1 for Cs2Np(U)O2Cl4 and 13128 cm−1 for Na(NpO2)[B5O8(OH)F]H2O, respectively. They correspond to the lowest CT excitations toward a configuration of σ1δ1ϕ1.23The CT transitions above 15000 cm−1 are much stronger. In Figure 2, it is shown that the strong peaks are vibronic side bands instead of ZPLs with energies listed in Table 1. Whereas the neptunyl CT vibronic transitions in Cs2Np(U)O2Cl4 and Na(NpO2)[B5O8(OH)F]H2O exhibit similarities in both energies and relative intensities. The f-f transitions do not have such a correlation. The energy level of the third excited 5f state varies from 7990 cm−1 for Cs2Np(U)O2Cl4 to 8845 cm−1 for Na(NpO2)[B5O8(OH)F]H2O. However, the

Figure 2. Experimental and simulated excitation spectra of Cs2Np(U)O2Cl4 at 75 K. The ZPLs of CT and f-f transitions are marked by the arrows.

fluorescence emission from the excited 5f state at 6881 cm−1 was monitored.20,28 The energies and relative intensities of the observed transitions in this spectrum correlate with those reported in well-resolved absorption spectra at 4.2 K14 The absorption spectrum of Na(NpO2)[B5O8(OH)F]H2O (Figure 3) was obtained at room temperature from a single crystal. The 8299

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Under the above conditions, an absorption spectrum of neptunyl CT transitions into the kth excited state with energy at EZPk can be constructed as a superposition of the ZPLs and the vibronic side bands arising from the origin and false origins (EZPk + ℏvjk) followed by a series of harmonic progressing bands with the frequency of the symmetric stretching mode ν1k.13,30

next excited 5f state does not exhibit such a large difference between the two systems. The significant difference characterizes the dependence of 5f states on ligand geometry and indicates that the energy levels of the 5f states are more sensitive to the crystal field. It is obvious in Figure 2 and Figure 3 that the vibronic peaks associated with the f-f transitions are also significantly different from that with the CT transitions.

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⎛ E + N ℏν1k ⎞ SkN Ik(E) = I0k exp( −Sk) ∑ ⎜ ZPk ⎟ EZPk ⎠ N! N ⎝

EMPIRICAL MODEL OF ACTINYL VIBRONIC TRANSITIONS As demonstrated successfully for uranyl in crystals and complexes,11,13 the modified Huang−Rhys theoretical model of vibronic transitions is quite powerful to provide a satisfactory simulation to the emission and absorption spectra of complicated structures. The success of this model suggests that its treatment of vibronic coupling for uranyl in crystalline compounds is fundamentally sound, and that the primary mechanisms of vibronic coupling do not change substantially for uranyl in different lattice (ligand) environments. In summary, the following basic assumptions and conditions are adopted into the empirical model. (1) The vibronic interaction is harmonicm, and local vibration modes are independent from each other, so that the Huang−Rhys theory of ion-lattice interaction can be applied. The ion-lattice coupling strength, known as Huang−Rhys parameter, is given by29 S=

νmΔ2 2ℏ

⎡ ⎢ exp ⎢ ⎢ ⎢⎣

(

−(EZPk + N ℏν1k − E)2 2σ12k

)

2πσ12k

⎛ −(EZPk + N ℏν1k + ℏνjk − E)2 ⎞ ⎤ ⎟⎥ exp⎜ 2σjk2 ⎝ ⎠⎥ + ∑ cjk 2 ⎥ 2πσjk j≠1 ⎥⎦

(2)

where I0k is an intensity constant proportional to the electronic cross section between the ground state and the excited state; Sk is the Huang−Rhys parameter, and σjk characterizes the line width which includes contributions from homogeneous broadening (temperature dependent) and inhomogeneous broadening (induced by defects). Intensity coefficients cjk are introduced to adjust the relative intensities between different modes.

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(1)

RESULTS AND DISCUSSION Absorption (excitation) spectra of the two neptunyl compounds were simulated respectively using eq 2. As shown in Figure 2 and Figure 3, the simulation is able to reproduce very well the experimental spectra. The spectra include five CT transitions and three f-f transitions. The ZPL energies, vibration frequencies along with the values of Huang−Rhys parameter and line width are listed in Table 1 and Table 2 in comparison

where Δ is the variation of the geometric parameter of the actinyl unit (commonly understood as the equilibrium position of the actinyl center), ν is the angular frequency of the vibration mode coupled to the final electronic state, and m is the effective mass of the vibrational mode. (2) Three intrinsic modes of actinyl play leading roles in CT vibronic coupling. These modes are the symmetric and asymmetric stretching modes (ν1, ν2) and the bending mode (ν3) of the linear molecule (OAnO). They are involved predominantly in giving rise to the major vibronic bands in actinyl optical spectra. The vibration modes associated with the ligands in the equatorial plane of the actinyl contribute significantly to vibronic transitions. Denning et al. studied the vibrational modes in the tetrachloride uranyl compound of Cs2UO2Cl4.9 With four Cl− ions in the equatorial plane, [UO2Cl4]2− forms an octahedron with D4h symmetry. There are 8 orthogonal vibration modes involving the Cl− ions in the equatorial plane. The frequencies of the ligand modes spread in two groups, one group with frequencies centered around 120 cm−1 and another with frequencies close to 250 cm−1 in coincidence with the OUO bending mode. Experimental results indicate that not all of these modes are coupled to the ZPLs. In addition, lattice modes with energies less than 100 cm−1 can be observed in actinyl spectra. (3) Only the symmetric stretching mode of the actinyl center is allowed by the selection rules of the Franck−Condon (FC) mechanism for coupling to electric dipole transitions of the actinyl ion; thus, multiphonon bands of this mode progress harmonically up to higher than five orders. Known as false origins, the non-FC modes including the intrinsic actinyl asymmetric stretching and bending modes are coupled to electronic transitions through the progressing FC mode and progress in the same frequency of ν1.

Table 2. Parameters of Vibronic Coupling in Neptunyl Compounds Na[(NpO2)B5O8(OH)F](H2O) (295 K) Cs2′NpO2Cl4 (75 K)

ν3(ν4)

ν6

νl

σCT

σf

250

120

70

110

160

245

139

70

12

70

with previous experimental results14,15 and theoretical calculations23 for the Cs2Np(U)O2Cl4 system. In simulating the low temperature spectrum of Cs2Np(U)O2Cl4 as shown in Figure 2, a great deal of the details is elucidated. Vibronic progressions up to seventh order are counted, and vibronic lines are resolved with a resolution of 10 cm−1 determined for best fitting the experimental spectrum. In comparison with uranyl in the same compound, the energy level of the lowest CT state of neptunyl decreases from about 20,000 cm−1 to 13,000 cm−1. This is because the energy gap between the valence band and the 5f2 configuration is smaller than that for the 5f1 configuration. This reduction is primarily independent of ligand environment. At the high energy side, the progressing CT and f-f vibronic transitions of neptunyl overlap uranyl transitions with ZPLs at 20096 cm −1 and 20390 cm−1. As shown in Figure 2, inclusion of these two uranyl bands enables the fitting of the 8300

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Cs2Np(U)O2Cl4 spectrum with energies above 20000 cm−1. As we pointed out in the previous section, the empirical simulation is based on the assumption that neptunyl electronic transitions couple selectively to the local and lattice modes. In addition to three intrinsic modes of the neptunyl, only two ligand modes, one near 139 cm−1 and another at 245 cm−1, and one lattice mode at 70 cm−1 are involved. Some minor peaks due to other ligand modes are not included in the simulated spectra. Others with frequencies very close to 139 cm−1, 245 cm−1, or 70 cm−1 are taken account by the parametrized line width. This approach apparently is effective in reproducing the experimental spectrum for the two neptunyl complexes in different geometries. It should be pointed out that the experimental spectrum shown in Figure 2 is composed from four segments recorded separately. The segments were normalized using the overlapped start and end sections of adjacent segments. Adjustment of the relative intensities of the lines in the beginning and ending regions of individual segments was made to obtain the smooth spectrum plotted in Figure 2. This adjustment does not affect the accuracy in frequency fitting but may impose uncertainties upon the intensity parameters. As we see in both Figure 2 and Figure 3, the CT1 and CT2 groups of CT bands are weaker than those in the higher energy region. Accordingly, the intensity parameter I0k varies significantly to correlate the group intensity changes. It is noticed that in Figure 2, among the vibronic bands of CT2, CT3, and CT5, the intensity of ν3 (ν4) with energy at 245 cm−1 is much stronger than that of the ν6 and νl. As a result, the intensity coefficient c3k is approximately 2.5 to 3 times c6k. The vibronic bands in the CT4 group do not have such a variation. Interpretation of the intensity changes between different vibronic bands requires more fundamental understanding of the electronic states and ligand dynamics. As a result of thermal broadening shown in Figure 3, ZPLs and the associated vibronic lines in the spectrum of Na(NpO2)[B5O8(OH)F]H2O recorded at room temperature are not visually distinguishable. Only the progression bands separated by ν1 are clearly resolved. Regardless of thermal broadening that obscures the fine structures of vibronic features, empirical simulation provides a quantitative interpretation to the f-f and CT vibronic transitions in the room temperature spectrum of Na(NpO2)[B5O8(OH)F]H2O. Except for the f-f transitions, the structures of CT vibronic transitions of neptunyl in both compounds have very similar characteristics that show a strong resemblance to the CT transitions of uranyl. As predicted by the theoretical model expressed in eq 2, all CT vibronic bands progress in a frequency of approximately 715 cm−1, which is assigned to the neptunyl symmetric stretching mode ν1. The frequency of the asymmetric stretching mode ν2 for the excited states is only 15 cm−1 higher than ν1. The 15 cm−1 difference between ν1 and ν2 is on the same scale of the line width; therefore, the lines of symmetric and asymmetric stretching modes are not resolved in the spectrum. The frequencies of the bending mode (ν3, coincident with Np−Cl sym. stretching mode ν4,) and the ligand modes undergo little change between different electronic states. Depending on temperature, all lines become much narrower at 75 K (Figure 2) than at the room temperature (Figure 3). For all the CT vibronic transitions in Cs2Np(U)O2Cl4, including the ZPLs and vibronic transitions, the simulation resulted in a line width of 12 cm−1. The line width in the room temperature spectrum of Na(NpO2)[B5O8(OH)F]H2O increases to above 100 cm−1.

Whereas the vibration frequencies and coupling constant do not vary significantly in the CT transitions, the difference between CT transitions and f-f transitions is large. As listed in Table 1, for Cs2NpO2Cl4, the value of ν1 varies from an average of 714 cm−1 for the CT transitions to 790 cm−1 for the f-f transitions. The value of S drops in average from 1.0 to 0.55. Similar changes in ν1 and S resulted for Na(NpO2)[B5O8(OH)F]H2O. The change in ν1 indicates that the ONpO bond length is shorter in CT states than that in a 5f dominated state. Accordingly, a longer neptunyl bond leads to weaker vibronic coupling, which means a smaller S. According to eq 1, the change in ONpO bond length can be calculated as ΔR(Np−O) =

2ℏS mν

(3)

where the effective mass m is the mass of two oxygen atoms and ν is the angular frequency of the symmetric stretching mode in the final electronic state. It should be pointed out that in a previous report13 the change of uranyl bond was calculated not using the angular frequency. Therefore the calculated value was larger by a factor of (2π)1/2. Using the values in Table 1 for the Huang−Rhys parameter (1.0 and 0.55) and the frequency of the symmetric stretching mode (710 cm−1 and 790 cm−1), respectively, for CT and f-f transitions, we obtained a bond change of 5.4 pm in CT transitions and 3.8 pm in f-f transitions. These values are in agreement with ab initio calculations23 but much smaller than those determined from an empirical model.10 Another noticeable difference between CT vibronic transitions and f-f vibronic transitions is that in the f-f transitions the lines are broader, and more local and lattice modes are involved. In fact, the apparent line broadening in f-f transitions can be attributed to coupling with more local and lattice modes. It is evident in Figure 2 that the f-f transition with ZPL at 17250 cm−1 is associated with a progressive (also in frequency of ν1) vibronic band with an origin approximately 500 cm−1 above the ZPL. There is no local mode with such a high frequency. Denning et al. attributed this group of lines to a progression of ν4, the neptunium-chloride stretch mode, which has a frequency of 257 cm−1 in the ground state.14 However, no higher order harmonics of such a progression appear in the spectrum. It seems more proper to assign this origin to the mixing of ν3 and ν4, the ONpO bending mode and Np−O (equatorial) stretching mode. The mixture of ν3 and ν4 further couples to ν1 and serves as a false origin of harmonic progression in a frequency of ν3 + ν4 + Nν1. As shown in Figure 2, this assignment fits the observed spectrum very well. The same assignment was made in the simulation of the spectrum of Na(NpO2)[B5O8(OH)F]H2O. As shown in Figure 3, excitation to the fourth 5f2 state (f4) has a ZPL energy at 8845 cm−1 and is associated to weaker vibronic side bands that stretch 1500 cm−1. This profile is similar to that of the f-f transition at 17250 cm−1 but has a much smaller value for the Huang−Rhys constant (S = 0.3), which is consistent with dominant 5f2 components in the wave functions of the ground state and the f4 state.

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CONCLUSIONS The empirical simulation of absorption and excitation spectra of neptunyl in two crystals of different structure and local geometry has provided a quantitative and fundamental understanding of neptunyl electronic transitions and their 8301

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vibronic coupling. It provides a powerful method to assign a complex vibronic spectrum that includes overlapped f-f and CT transitions. Ambiguous assignments are eliminated through systematic fitting of the intensities as well as the energies of transitions predicted by theoretical calculations. It is shown that the electronic energy levels of CT states and vibronic coupling are not sensitive to ligand environments. The chemical structure and ligand coordination are quite different between CsNpO2Cl4 and Na[(NpO2)B5O8(OH)F](H2O). However, as shown in Table 1, the energy variation for a given CT state is only on the order of 1.5%. The values of neptunyl stretching frequencies and vibronic coupling constant (Huang−Rhys parameter) undergo no significant changes between the two compounds. These results indicates that the CT states are localized in the neptunyl ion, where the interactions with the ligand ions including those nearest neighbors in the equatorial plane only make minor contributions. A significant difference between the CT transitions and f-f transitions is identified. It is shown that the electronic transitions involving states in primarily the nonbonding 5f orbitals exhibit a much weaker vibronic coupling, whereas the crystal-field contribution to the energy levels of the excited electronic states is much stronger than that in the CT states. The present analysis gives a direction for further theoretical investigation of the electronic structure and interaction mechanisms that determine the properties of vibronic coupling in actinyl compounds.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge beneficial discussions with W. H. E. Schwarz. Work performed at Argonne National Laboratory was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences, under contract DE-AC02-06CH11357. We are grateful for support provided by the U.S. Department of Energy, Heavy Elements Chemistry Program, under Grant DESC0002215 (T.A.S.).

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dx.doi.org/10.1021/jp302679q | J. Phys. Chem. A 2012, 116, 8297−8302