Resonance Raman excitation profile for uranyl acetate in dimethyl sulfoxide

Spectrochimica Acta Part A 55 (1999) 1337 – 1345

Resonance Raman excitation profile for uranyl acetate in dimethyl sulfoxide Takeshi Soga *, Ken Ohwada Department of Material Science, Japan Atomic Energy Research Institute, Tokai-mura, Ibaraki-ken 319 -11, Japan Received 14 February 1998; accepted 5 October 1998

Abstract The intensity of Raman lines of uranyl acetate/UO2(CH3COO)2 in dimethyl sulfoxide has been measured as a function of excitation energy in the region of the Laport-forbidden f – f electronic transitions, using ten output lines of an argon-ion laser. The resonance Raman excitation profile of the totally symmetric stretching vibration of the uranyl observed at 829 cm − 1 shows vibrational fine structure, which resembles the vibronic structure of the electronic absorption spectrum but does not completely coincide. Experimental excitation profile is compared to that calculated using transform theory with parameters derived from the observed absorption spectrum of the resonant electronic state of interest. The non-Condon model (generalized B, C-term source of scattering) gives relatively good agreement with experimental results. The disagreement between the experimental data and the calculated resonance Raman excitation profile may be ascribed to interference between the weak scattering from the neighboring forbidden electronic states and strong preresonance scattering from allowed electronic states at higher level. The change in the ˚ . © 1999 UO equilibrium bond length resulting from the 1Sg+ “ 1Fg electronic transition is found to be 0.068 A Elsevier Science B.V. All rights reserved. Keywords: Excitation; Raman; Resonance; Uranyl acetate

1. Introduction Resonance Raman spectra are obtained when a molecule is excited with a laser beam whose frequency corresponds or closely corresponds to an electronic transition frequency of the molecule. Such spectra are frequently characterized by an enormous intensity enhancement of the band arising from a totally symmetric fundamental vibration of the molecule, together with appearance of * Corresponding author.

high intensity overtone progressions for the same fundamental vibration, especially in the case of an electric-dipole-allowed transition. In such studies, it has been realized that the dependence of the Raman intensity on the wavelength of exciting light, i.e. the Raman excitation profile (REP) for the scattered mode, as well as the electronic absorption spectrum (ABS), can provide useful information on the character of resonant excited states. Therefore, the analysis of the REP is indispensable for understanding the resonant excited electronic state of interest. Existence of a close

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theoretical relation between the ABS and the REP, which are the quite different experimental observations, has been known for many years, and the relation emerged recently in the form of useful transform methods for determining REPs from observed ABSs. By introducing the adiabatic approximation along only one normal coordinate identified with the vibrational state being examined in the Raman experiment, Stallard et al. [1] derived a powerful transform law for the Stokes scattering of a totally symmetric vibration. The great advantage of the transform method is that, despite the fact that the Raman excitation profile is a multimode-dependent function, only parameters associated with the particular vibrational mode under consideration are used. The transform technique have been proven very useful in analyzing the resonance Raman spectra of molecules and crystals that contain many modes of vibrations [2 – 5]. Recently, we have obtained the REP of the uranyl symmetric stretching vibration at 835 cm − 1 of uranyl nitrate in dimethyl sulfoxide and have found that the REP resembles the vibronic structure of the electronic absorption spectrum [6]. In this paper, we shall pay attention to the uranyl acetate-dimethyl sulfoxide system to examine the ligand effect on the REP in comparison with the uranyl nitrate-dimetyl sulfoxid system [6] (the bond between CH3COO − group and uranyl is less ionic than that between NO3− and uranyl) and to estimate the equilibrium conformation of the uranyl ion in the excited state 1Fg. Transform methods are applied to analysis of the observed REP obtained in resonance with the excited electronic state.

Tokyo, Japan) was dissolved in DMSO of analytical reagent grade without further purification. The 0.48 and 0.048 M UO2(CH3COO)2 –DMSO solutions were used in the Raman spectral and the electronic (vibronic) absorption measurements, respectively.

2. Experimental

2.3. Electronic absorption spectral measurements

2.1. Preparation of sample solutions

The absorption spectra of the solution contained in the 5-ml quartz cell (10 ×10× 50 mm) were measured at room temperature in the region from 300 to 700 nm with a spectrophotometer (Japan Spectroscopic Ubest-50). No change was observed in the UV and visible absorption spectra before or after long exposure to the laser beam.

Dimethyl sulfoxide (DMSO) was chosen as the solvent for the spectral measurements, because its Raman lines can be conveniently used as internal intensity standards. The stoichiometric amounts of uranyl acetate, UO2(CH3COO)2 (guaranteed reagent grade, Koso Chemical,

2.2. Raman scattering measurements The Raman spectra were recorded within the region of 4000 cm − 1 using the excitation lines at 528.7, 514.5, 501.7, 496.5, 488.0, 476.5, 472.7, 465.8, 457.9, and 454.5 nm with an argon ion laser (Spectra Physics Model 168-11/265). The laser power ranged from 100 to 150 mW. The beam was passed through the sample solution contained in the 5 ml single-path quartz cell (10×10×50 mm) at room temperature. Scattered light was collected in some small solid angle around an observation direction at 90° to that of the incident light using an f/1.2 lens and focused on the entrance slit of a 1 m double monochromator (Japan Spectroscopic CT1000D), equipped with holographic gratings and a scanning control unit (Japan Spectroscopic SMD-80). Detection was carried out by a photomultiplier tube (Hamamatsu Photonics R-464), coupled with a two-phase lock-in amplifier (NF Electronic Instruments 5610B). The frequencies quoted are accurate to 1 cm − 1. The Raman line at 1044 cm − 1 of DMSO was used as an internal intensity reference for Raman scattering of the uranyl symmetric stretching vibration, because DMSO does not decompose with long exposure to the laser beam and shows no resonance Raman effect [6].

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Fig. 1. Electronic absorption spectrum of the uranyl acetate – dimethyl sulfoxide system at room temperature. Peaks at 492.5, 476.0 and 460.5 are attributed to the 1Sg+ “ 1Fg transition. The vertical arrows indicate the laser lines used for excitation of Raman spectra.

3. Results and discussion The ABS of uranyl acetate in dimethyl sulfoxide measured at room temperature is shown in Fig. 1, together with the wave lengths of laser lines indicated by arrows. In the ABS, the uranyl ion has an intense structureless (electric-dipole-allowed electronic transition) absorption band in the shorter wavelength region (330 – 375 nm) and a series of medium intensity bands, Laport-forbidden f–f electronic transition in the wavelength region from 370 to 500 nm. These are changed by the host matrices around the uranyl ion, such as CH3COO − , NO3− , and DMSO. The vibronic structure of the latter band is subdivided into two

categories [7]: (I) the absorption situated between 500 and 445 nm consists of three well resolved peaks at 492.5, 476.5, and 460.5 nm attributed to the 1Sg+ “ 1Fg transition, and; (II) the absorption situated between 445 and 370 nm consists of unresolved peaks of the 1Sg+ “ 1Dg transition. The intensity of the Raman lines of uranyl acetate, UO2(CH3COO)2 in the DMSO solution has been measured as a function of exciting energies (ten output lines of argon-ion laser) in the vicinity of vibronically allowed electronic transitions ranging from 500 to 445 nm (1Sg+ “ 1Fg transition). The Raman spectrum observed with the 514 nm line of an argon-ion laser, which is the most frequently used exciting source, at room

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Fig. 2. Resonance Raman spectrum of uranyl acetate in dimethyl sulfoxide below 2000 cm − 1, observed with exciting light of 514.5 nm. The Raman lines due to the uranyl ion are indicated with arrows in the figure. The weak broad band (asterisked) around 1600 cm − 1 may be assigned to the bound acetate group, the remaining lines to DMSO.

temperature is shown in Fig. 2. The Raman lines due to the uranyl ion, in the UO2(CH3COO)2 – DMSO system, are indicated with arrows in the figure. The linear uranyl group, OUO would of course be expected to show two Raman active fundamental frequencies at about 830 and 200 cm − 1 which have been assigned to the totally symmetric stretching and bending vibrations respectively [8]. The two bands at 829 and 206 cm − 1 can be readily assigned to the two Raman active modes of the uranyl symmetric stretching and bending vibrations, respectively [6]. The weak broad band around 1600 cm − 1 (indicated by an asterisk) may be assigned to the bound acetate group and the remaining lines to DMSO. No Raman band was observed in the overtone and combination regions. This point will be discussed later in relation to the resonance interaction between the exciting laser line and the vibrational levels in the excited electronic state. Fig. 3 illustrates the enlarged view of the uranyl symmetric stretching vibration at 829 cm − 1 for three different wavelengths of excitation (454.5,

476.5, and 514.5 nm). From this one can see that the intensity of the Raman line at 829 cm − 1, change markedly depending on the wavelength of excitation light. The REP for the uranyl symmetric mode is a convenient means of showing this correlation qualitatively. In order to obtain the accurate experimental REP, one needs to know accurate scattering intensities of an objective Raman mode. However, the experimental determination of such intensities is commonly difficult, particularly when the resulting intensities are to be placed on an absolute scaler. For this reason, we here employed relative intensities of the uranyl totally symmetric stretching mode (829 cm − 1) to the internal standard of DMSO (1044 cm − 1), which show no resonance Raman effect. The closely spaced peaks at 1044 and 954 cm − 1 (due to DMSO) and 829 cm − 1 (due to uranyl) were well resolved (Fig. 3) and their band intensities were determined as the products of peak height and full-width-at-half-maximum. The relative intensity was determined as the average of five runs for each excitation line. The absorption of excit-

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Fig. 3. The enlarged view of the resonance Raman scattering in the UO2(CH3COO)2 – DMSO system around the uranyl symmetric stretching vibration at 829 cm − 1. The line at 1044 cm − 1 due to DMSO can be used as an internal intensity standard.

ing light and reabsorption of scattered Raman light by the sample solution itself were not taken into account, as it is rather difficult to evaluate these effect accurately. The REP for the uranyl symmetric mode was obtained by plotting the values of such relative intensities against the exciting wavelengths, and shown in Fig. 4 as filled circles. It is found from Figs. 1 and 4 that the observed REP of the uranyl totally symmetric stretching line has similar structure to the vibronic structure of the ABS, but does not superimpose on it. The same tendency is also found in the

systems of UO2(NO3)2 –DMSO [6] and RbUO2(NO3)3 –DMSO [8]. This phenomenon may be explained by the interference effect which has been pointed out by Friedman and Hochstrasser [9]. It is seen from Fig. 4 that the relative intensity change in the REP, ranging from 0.3 to 0.9, is not so large in comparison with that of the UO2(NO3)2 –DMSO system (0.25–1.25) [6]. This suggests that the REP of the uranyl–DMSO system depends considerably upon the ligands bound to the uranyl group. But, the ligand effect on the

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Fig. 4. Resonance Raman excitation profile for the 829 cm − 1 fundamental vibration. Filled circles give relative scattering intensities for the 0.48 M UO2(CH3COO)2 –DMSO solution with respect to the DMSO 1044 cm − 1 mode. The solid line gives the best transform fit with the experimental data within the non-Condon model using excited state frequency of 705 cm − 1.

observable Raman shift is small, i.e. there is a little difference (6 cm − 1) between UO2(NO3)2 – DMSO and UO2(CH3COO)2 – DMSO. While, the REP of RbUO2(NO3)3 – DMSO, in which three NO3− are bonded around the uranyl ion, displays a similar change (0.25 – 0.8) with that of UO2(CH3COO)2 – DMSO, and little difference (5 cm − 1) is seen in the observed Raman shifts between them. These results are summarized in Table 1. It is of interest to interpret the observed REP from a theoretical point of view. The interpretation of REPs is complicated in general, since a large number of phenomena can influence their behavior. One of the most serious difficulties in the calculation of REPs is so-called multimode

problem, intrinsic to the theory of the resonance Raman effect of polyatomic molecules. In this respect, the transform method allows one to circumvent this difficulty by transferring the multimode information contained in the ABS directly to the REPs [5]. For convenience the transform method as outlined by Stallard et al. [1] is used, though any other variant of the transform would be equally valid. The basis for the transform relation between ABS and a given REP lies in the Kramers–Heisenberg formulation for the off-diagonal molecular polarizability. In general, it is not easy to calculate the resonance Raman scattering tensor because of many modes which participate in the nuclear motions. Stallard et al. have derived a transform relation directly from the

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Table 1 The wave number (cm−1) of the Raman active vibration of the OUO totally symmetric stretching mode in both the ground (gv\) and excited (ev\) states and the change in the REP (DREP) for uranyl compounds in DMSO Complex

gv\ (cm−1)

ev\ (cm−1)

DREP

Reference

UO2(NO3) RbUO2(NO3)3 UO2(CH3COO)2

835 834 829

703 740 703

0.25–1.25 0.25–0.80 0.30–0.90

[6] [8]

a

a

Present work.

frequency domain Kramers – Heisenberg expression. Their approach is based on the recursion relation for Raman Franck – Condon factors and introduces the relaxation of several of the standard constraint. In particular, a limited adiabatic approach is used in which only the mode being examined in the Raman experiment is assumed to be factorized from the molecular eigenstate space, and the following useful transform equation for the Stokes scattering of a totally symmetric vibration has been derived:

    d sR 4 d sR − d V 3 d V

Þ

(v− va)4 = S (1 + Ca)fv −(1 −Ca)f(v − va)2 16C 2p 2 a (1)

% (Rrs )IO Ca =

S 1/2 a

 

.

 

d sR d sR and denote the orientad V dV Þ tionally averaged total resonance-Raman differential cross-section for totally symmetric modes with the scattered light polarized parallel ( ) and perpendicular (Þ) to the polarization of the incident light, respectively. v is a particular laser excitation frequency and va the vibrational frequency in the excited electronic state. The dimensionless relative non-Condon parameter Ca, a pure real number, represents the ratio at the amplitude level of the generalized B, C-term to A-term sources of scattering for the single factored vibration [13]. This parameter can be varied to provide a fit to Here,

      d sR 4 d sR − d V 3 d V

Þ

2 (v− va)4 IO = % (R ) f(v)+f(v−va) 2 rr 16C 2p 2 r

where,

r

the experimental REP. Sa = (kaDQa/2F( a)2 is the Franck–Condon coupling parameter (FC), where ka is the force constant, DQa is the displacement of the excited potential from that of the ground state, and F( a is the root mean squared zero point force. Sr (Rrr )IO is the vibronic coupling parameter. Eq. (1) as outlined above is appropriate to the description of resonance Raman scattering intensities of totally symmetric vibrational modes for excitation to an electric-dipole-allowed transition. In the present case, the definition Ca can not be used because the electronic transition (1Sg+ “ 1Fg) we are dealing with is mostly vibronic. Introducing an assumption that Sr(Rrr)IOˆSa1/2, one may modify Eq. (1) as follows [8].

(2)

The Kramers–Kronig transform of the ABS gives the real part of the polarizability, while the optical theorem shows how the absorption function itself represents the imaginary part [18]. Thus the complex linear polarizability f(v) at the frequency v can be calculated from the measured ABS through the Hilbert transform: 1 f(v)= P p

&

+

−

sABS(x) sABS(v) d x+ i x(x− v) v

(3)

Where, P denotes the Caushy principal value of the integral. The expected REP is now fully determined by the observed ABS and the excited electronic excited state fundamental vibrational frequency of interest through Eqs. (2) and (3).

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We calculated the REP for the uranyl symmetric stretching mode at 829 cm − 1 of UO2(CH3COO)2 in DMSO by use of this transform method and compared it with the experimental REP (Fig. 4). For the calculation, we have here assumed that the line shape function for the ABS in Eq. (3), sABS(v) may be expressed as some linear combination of Lorentzians, L(v)n, sABS(v)=m n = 1 gnL(v)n where the coefficients gn are determined so as to give the best agreement between the calculated and experimental ABSs. In the present case, the number of Lorentzian is m = 13. The only adjustable parameter in Eq. (2) for resonance exciting Raman line shape fitting is the excited electronic state fundamental vibrational frequency of interest, va. It may be varied within a limited range of the known ground state frequency, if the excited value is not independently available. We could properly estimate this parameter from consideration of the observed ABS (Fig. 1) as shown below. The maxima in the electronic absorption spectrum of the uranyl ion can be expressed by the following equation [10]: nabs =nE +nsns + nAnA + nBnB + Sinini, where nE refers to the wavenumber of the pure electronic transition, nS to the symmetric stretching vibration, nA to the antisymmetric one and nB to the bending vibration. ni concerns all ligand or crystal lattice vibrations coupled to the electronic transition. The number nS, nA, nB, and ni give the number of quanta of the various vibrations involved. According to this formula, a uranyl spectrum should be very complex. It has however been established experimentally that the spectrum is simplified because one vibration is predominantly coupled to the electronic transitions of the uranyl ion. Several authors [11] have identified it as the symmetric stretching vibration nS, and have described the ABS by nabs = nE + nSnS. From the observed ABS (Fig. 1), we can see that intervals between these vibronic maxima are separated with almost regular intervals, 704 and 707 cm − 1 in the electronic transition region of 1 + Sg “ 1Fg. The only adjustable parameter, va was chosen as 705 cm − 1, the mean value for two intervals.

Using Eqs. (2) and (3), we have calculated the REP for the OUO stretching mode at 829 cm − 1, in which the intensities are adjusted to those of the experimental REP. In Eq. (3), though the limits of the integration extend over all frequencies, the finite extent of the ABS allows truncation, 18.5–22.5 ×103 cm − 1. In Fig. 4, the calculated REP for the 829 cm − 1 mode is compared with the experimental REP within the region of the electronic transition 1Sg+ “ 1Fg (550–450 nm; 18.5–22.5 ×103 cm − 1). In this figure, filled circles indicate the experimentally observed relative intensities and the solid line gives the best transform fit in the non-Condon model using the excited state frequency of 705 cm − 1. One can see from the figure that an excited frequency of 705 cm − 1 in the non-Condon model gives relatively good agreement with experiment except for the low frequency region below 20.5× 103 cm − 1. The discrepancy between the experimental and calculated REPs are ascribed to interference between the scattering from a neighboring vibronic resonance electronic state (1Sg+ “ 1 Dg) and strong preresonance scattering from allowed electronic states of higher energy, situated between 27 and 30× 103 cm − 1. Considering the experimental ABS in Fig. 1, one can see that the intensity of resonance Raman scattering in the region of Sg+ “ 1Dg may be more strongly enhanced than in the region of the 1Sg+ “ 1Fg electronic transition. The preresonance scattering due to the allowed transition may also be stronger than that of these two forbidden electronic states. In order to understand the experimentally observed REP correctly, we will have to investigate the REP resulting from these three electronic states. In the case of an electric-dipole-allowed transition, the Albrecht’s A-term predominantly contributes to a Raman spectrum, and the high intensity totally symmetric fundamental mode and its overtone progressions are expected to be observed. The fact that there are no overtone and combination bands at all in this system means that the resonance Raman intensities of the uranyl symmetric stretching mode depend mainly on the generalized B, C-term (non-Condon term) source of scattering [12].

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The scattering intensities of the uranyl totally symmetric stretching vibration at 829 cm − 1 depend on the wavelengths of exciting laser lines. Application of an empirical rule of Hirakawa and Tsuboi [13– 15] to the present case suggest that the uranyl ion in the excited electronic state has an equilibrium conformation linearly distorted along the symmetric stretching mode. It is of importance to examine the equilibrium geometrical change in the uranyl bond length in the excited electronic state of interest. In order to evaluate the change in the uranyl bond length in going from the ground to the excited electronic states, 1Sg+ “ 1Fg, a simple rule presented by Badger [16], which relates a bond length with its stretching force constant, was employed. This simple rule has been proven very useful for analyzing the UO bond length RUO in the ground state uranyl compounds from their stretching force constant KU − O determined by normal coor1/3 dinate analysis: RU − O =1.08K − U − O +1.17 [17]. If we would expect this relation be applicable to the excited electronic state vibrational modes, the difference in the UO bond lengths between the ground and electronic excited states is written by 1/3 − 1/3 DRU − O = rU − O −RU − O =1.08(k − U − O −K U − O), where rUO and kUO denote the UO bond length and the symmetric stretching force constant in the excited electronic state, respectively. The stretching force constants in the ground and excited states can be obtained from normal coordinate analyses. The wave number in the ground state is taken to be 829 cm − 1 in the observed Raman shift, while that in the excited state is obtained as 705 cm − 1 from the vibronic progression of the experimental ABS. The calculated increment of ˚. the UO bond length is about 0.068 A

4. Summary The REP of the totally symmetric OUO stretching mode observed at 829 cm − 1 in the UO2(NO3)2 – DMSO system was measured using ten output lines of an argon ion laser at room temperature. It has been found that the REP for .

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the O–U–O stretching mode is similar to the vibronic progression of the observed ABS and displays maxima in the wavelength region of interest. The intensity change in the observed REP is not so large as that of the UO2(NO3)2 –DMSO system. Transform methods have been applied to the analysis of the observed REP. The non-Condon model gives relatively good agreement with the experimental REP. Differences between the experimental REP and the transform predictions may have resulted from the coalescence of the neighboring vibronic resonance state and the preresonance scattering due to allowed electronic transition located in the shorter wavelength of visible spectrum. Elongation of the OUO bond arising from the 1Sg+ “ 1Fg electronic transition was estimated.

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