High-resolution infrared study of the ν2 + ν3 band of CF3I

JOURNAL

OF MOLECULAR

SPECTROSCOPY

147,208-214 (199 1)

High-Resolution Infrared Study of the vz + v3 Band of CF31 A. BALDACCI, S. GIORGIANNI, P. STOPPA, Diparlimennto

di Chimica-Fisica,

Univrrs~rd

A. DE LORENZI, AND

di Venezia,

D. D. 2137, l-30123

S. GHERSETTI Venezia,

Ital).

The gas-phase infrared spectrum of CFJ has been studied in the v2 + V)region between 1030 and 1035 cm-’ with a resolution of about 0.002 cm-’ employing a tunable diode laser spectrometer. The measurements have been performed at low temperature (1230 K) and the k’ structure in many R(J) manifolds has been resolved and analyzed. The band has been found perturbed by V?+ v, through Coriolis resonance and the maximum J and K values reached in the analysis were 62 and 30, respectively. The observed transitions have been fitted ( CJY 13 X 10m4cm-‘) to the excited state parameters up to the quartic coefficients and a set of effective molecular constants for the V) + uj band has been determined. The J structure of residual weak features due to Q Academx + 2~ - vj has also been identified and spectroscopic constants have been derived. (~11991 Press. Inc.

1. INTRODUCTION

The infrared spectrum of CFJ has been extensively investigated in recent years and a number of spectroscopic parameters have been determined from the analysis of selected regions dealing with fundamental, overtone, and combination bands ( l-4). Interest concerning this molecule mainly has its origin in the presence of strong features around 9.4 pm, in the range of COZ laser lines emission which allows to study multiphoton excitation processes (5. 6). Although this large absorption predominantly arises from the symmetric CF3 stretching vibration v ,, the presence of different interacting rovibrational states 2v 5, us + 2~. v3 + 3vg, v2 + v3, v, + v6, etc.. causes severe problems in interpreting the complex spectral structure ( 7. 8). The v? + v3 combination band has been already investigated under medium resolution and the J structure in the P and R branches has been analyzed by a polynomial method ( 9). Due to the higher resolution provided by a diode laser spectrometer, the individual lines within each multiplet have now been resolved and the fine K structure has been attributed. In order to make reliable the assignment, the interference of the hot band absorptions was depressed recording the spectra at about 230 K. This contribution deals with the interpretation of the observed features and the determination of the molecular constants of v2 + v3: this level was found to be in interaction by Coriolis resonance with the v2 + vg state. Results from the analysis of the hot band vz + 2v3 - v3 are also given. II. EXPERIMENTAL

DETAILS

High-resolution spectra of CFJ (purity > 99%) have been recorded in the range 1030-1035 cm-’ using the tunable diode laser spectrometer at the University of Venice. The measurements taken at a pressure around 1.5 Torr were carried out at about 230 0022-2852/91 $3.00 Copyngbt D 1991 by Academic Press, Inc. All rights of reproduction rn any form resewed.

208

u2 + vj BAND

309

OF CFJ

K with a homemade 49-cm-long stainless steel cell cooled by flowing liquid nitrogen vapors. Absolute wavenumber calibration was provided by well-known NH3 absorption lines (IO), while a 2.59-cm germanium Ctalon with a fringe spacing of about 0.0476 cm-’ was employed for the relative calibration. The absolute wavenumber accuracy is estimated to be better than 0.003 cm-’ and the internal consistency within the same laser mode is of the order of 0.00 1 cm-‘. III. DESCRIPTION

OF THE

SPECTRUM

AND

PERTURBATIONS

The CFJ molecule, belonging to the C,, point group, is a prolate symmetric rotor and the uZ + v3 vibration of A, species produces a parallel-type band. The region investigated deals with the R branch of this combination and the fine structure within the R (J,K) clusters examined (J < 62 and K G 30) has been resolved and properly assigned. The individual transitions exhibit the expected intensity alternation which. according to spin statistic considerations, gives lines from ground state levels with K = 3n twice as strong as the others. As an example, Fig. 1 shows the distribution of the K components in the R( 54) and R(55)multiplets, where the marked degradation to lower frequency is mainly related to the negative value of ( aB - 01.~ ) . As can be observed, the cold spectra are particularly important for heavy molecules, such as CF31, having low frequency vibrations: the rotational structure arising from the relatively strong hot band absorptions at room temperature can indeed distort the involved pattern of the main band. The displacement of the K structure in the multiplets analyzed is different from

b

cm

I 1033 5724

I. R-branch portion (P 2 1.5 Torr ) of CF,I v2 + v3 band near 1033.6 cm-’ : K assignments of R(54 ) R(55 ) multiplets are indicated. (a) Spectrum recorded at 2230 K. (b) Spectrum recorded at 2298 K.

FIG.

and

-1

BALDACCI

210

ET AL

that expected for an unperturbed parallel band and the observed irregularities clearly reveal that the uz + IJ~band as a whole is affected by perturbation. In this region the u? + Q, combination, particularly close in energy to ~1 + u3, is also expected to occur and a Coriolis X, y type ( Al = Ali = + 1 ) interaction between these two states can give a reasonable explanation of the irregular observations. Interestingly, the components u3 and ug of this dyad are themselves coupled through Coriolis resonance, which ptoduces a strong effect on the molecular constants of the involved levels ( I1 ). The spectral position of the Q + Vgband is far to be known with certainty. Since the Vg fundamental is estimated to be between 260 and 266 cm-‘, combining these values with that of u2 + ug - u6 at 742.618 cm-’ (12) a frequency range of 1003-1009 cm-’ for the u2 + ug band origin is obtained. A very recent Raman study of CFJ in liquid xenon solution locates the u3 and ug vibrations at 286 and 267 cm-‘. respectively ( 13). Since the frequency of the u3 band is in good agreement with the infrared value at 286.303 cm-’ of Ref. (9). similarly, a value of the Vg vibration around 266 cm-’ seems to be more likely; on this basis, the uz + u6 should be placed at about 1009 cm-‘. Unperturbed excited state parameters of v2 + u3 and uz + u6 were obtained from results of Refs. (9. II, 12) assuming additivity in the CY.“%~ coefficients; the effective COIiOh COnStant (A$&, was supposed t0 be equal t0 (A!&. while the u, + ug band origin was estimated to be around 1009 cm-’ taking into account for the ug vibration the Raman value of Ref. (13). By considering the reduced energy levels of the states involved, the Coriolis perturbation appears more important between 1vz + v3, k) and 1~ + u6f1, k + 1) since the levels Iv2 t u3, k) and Iuz + ug’, k - 1) are rather far apart. Because of the interaction, the u1 + u3 levels are pushed upward and no crossing with the uz + u6 levels is expected limitedly to the J and K values of the transitions identified (J i 62 and K =S 30). Due to a very low energy difference between IQ + I’~ and I+ + uh with respect to IQ and Vg(20.7 cm-’ Ref. ( I,?)), almost the same effect of resonance should be observed in the present system: hence. a near equality of the Coriolis coupling constants {zi.2h and
AND

DISCUSSION

The individual transitions for a parallel band can be represented by suitable relations derived from the energy expression of the rovibrational levels given by E( u, J, K) = E, + (A, - B,)k-2

+ B,J( J + 1) - D;J’(J+

when the centrifugal

distortion

l)‘-

D&J(J+

l)K’-

terms up to the fourth order are included.

D;K’

(I)

v? + vj BAND

OF CF,I

311

The rotational analysis was carried out following standard techniques and a computer program, which performs a least-squares fit of the assigned transitions to produce a set of molecular constants, was employed. Preliminary line position calculations were performed using or deriving the spectroscopic parameters from Refs. ( 9, 11, 12). Line assignments were established by also considering the expected intensity alternation and, once the excited vibrational state parameters were refined, the analysis proceeded iteratively with further assignments, revision of constants, and prediction of new transitions. After excluding the blended lines, a total of 360 transitions were correlated in the final least-squares fit (u N 13 X 10m4 cm-‘); the observed frequencies along with the (0 - C) values are listed in Table I while the effective spectroscopic constants for the vz + u3 band are displayed in Table II. In the refinement procedure of the molecular parameters, the ground state constants were constrained to the microwave values ( 11) and only the band origin and the upper states constants were allowed to vary. The very small difference (Dk - Dx) could not be significantly determined and therefore it was fixed to zero in the final least-squares fit. The values of the parameters reported in Table II include the contribution from the Coriolis coupling constant {23,26and one of the resonance effects, as it can be seen. is reflected in the large difference of DJ~ between the ground and the upper states. The effective constant ol& = 74( 1) X lop6 cm-’ is very close to the deduced value of 71 X 10e6 cm-’ assuming additivity of CY~(12) and CE?(11): of course, for the latter parameter the effective (32.2 X lo-’ cm-l) rather than the unperturbed ( 126 X 1O-m6 cm-’ ) value has to be taken into account. The satisfactory agreement between the determined and the derived value of cyf3 furthermore proves that the v1 + IQ vibration is mainly perturbed by u2 + !+, through Coriolis resonance and possible contributions from additional interactions with close-lying vibrational states should be expected to be very small. Besides the transitions of the IQ + u3 combination. the region investigated exhibits additional weak lines coming from hot bands involving v3 or &, in the lower vibrational state. The weakness of the last features did not allow any determination of the fine structure and the rotational analysis has been only limited to the J peaks of the u? + 2v3 - v3 band. The observed frequencies were fitted to the well-known equation

I?‘.~ = u + bm + cm2,

(2)

where a = Vg + [(,4’ -A”) b = B’ + B”;

- (B’ - B”)]K’

c = B’ - B” - (D; ~ Dl;) N -atl.

The inclusion of higher powers of m in the polynomial did not improve the fit. For the observed absorption maxima usually one assumes that the related frequencies correspond to a certain value of K and therefore a correction for the displacement from K = 0 should be applied. Assuming additivity of (~~3~and considering the max.. imum of the J peaks for K = 3, the quantity [(A’ - A”) - (B’ - B”)] K’ is z-9 X lop4 cm-’ . In this particular case the correction for each R(J) multiplet should be calculated individually since the displacement between the absorption maximum at K = 0 depends significantly also on the J value. An interesting feature is provided by

212

BALDACCI

ET AL.

TABLE I Observed

R(J. K) Line Positions (cm-‘)

of v2 + v3 Band of CF,I

213

the shape of the R( 58) and R( 73 ) manifolds depicted in Fig. 2. At low J values the K structure is red-degraded and, as J gradually increases, it becomes first more compressed and later blue-degraded. This phenomenon can be understood if the K dependent term is considered in the equation for the transition frequencies. Within a cluster the position of each line with respect to the K = 0 transition is expressed by v[R(J.

K) - R(J,

0)] = [(AA - AB) - 2(J + l)DTn - (J + 1 )(J + 2)AD,,]K’

- AD,K4.

(3)

For J N 70, the positive value of - (J + 1 )( J + 2) ADJti becomes comparable with the negative values of ( A,4 - AB) and -2 (J + 1) Dl;li, and therefore for high J values the K’ term will be positive causing a blue degradation of the K structure, The observed R(J) peaks (see Table III), taken at the maximum absorption without correction for the displacement from K = 0, were used in the least-squares fit and the

R(X)

R1401

1030

I

8736

cm-’

v

FIG. 2. R-branch sections (P v 1.5 Torr. T 4 298 K) of the CF 31 ~2 + 2~3 - q band showing opposite degradations in the R( 58) and R( 73) multiplets. Manifolds of the “2 + v) band (R( 26) and R( 40)) are also indicated.

214

BALDACCI

ET AL.

TABLE III R(J)

Lines and Molecular

Parameters

(cm-’

) of “2 + 2~ - v3 Band of CFJ

obtained molecular parameters are included in the same table. Application of the additivity rule for ~1~ of the hot band v2 + 2v 3 - v3 roughly indicates that the upper state is also perturbed by Coriolis interaction. In conclusion? from this investigation a set of effective constants up to the quartic coefficients for the v2 + v3 combination as well as some parameters for the hot band involving v3 in the lower vibrational state were obtained. The present results are in satisfactory agreement with those of Ref. (9), but are more complete and determined with a greater accuracy. The observed irregularities in the rotational structure of the u2 + v3 band have been interpreted on the basis of Coriolis resonance with the u7 + v6 vibrational state. ACKNOWLEDGMENT This work was supported

RECEIVED:

December

in part by funds from MURST.

Roma (40% program).

26, 1990 REFERENCES

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