Diode laser spectrum and rovibrational analysis of the ν2 + ν5 band of CF3Cl

JOURNAL

OF MOLECULAR

SPECTROSCOPY

154,265-276 (1992)

Diode Laser Spectrum and Rovibrational Analysis of the v2 + v5 Band of CF&I S. GIORGIANNI, P. STOPPA,A. DE LORENZI, AND S. GHERSETTI Dipartimento di Chimica Fisica, Universitri di Venezia, D.D. 2137, I-30123 Venezia, Italy

The vibration-rotation spectrum of the ~2+ Yeband of CFQ, with natural isotopic abundance, has been investigated in the range 1332- 1360 cm-’ with a resolution of about 0.002 cm-’ using a tunable diode laser spectrometer. The measurements have been performed by keeping the sample at low temperature (~240 K) to reduce the interferences arising from hot bands. Both J and K structures have been resolved and analyzed in the ‘$(P, (2, R) subbranches, and many individual transitions covering the ranges -28 =ZKAK s 24 up to J = 68, and -18 < KAK G 24 up to J = 58, have been identified for CFr”Cl and CF3”Cl, respectively. The perturbation effect due to the Al = Ak = *2 resonance has been found significant even in other transitions with low K values and not only for the kl = 1 level. Excited state parameters up to the quartic terms have been determined for the Ye+ V)band of both isotopic varieties from a fit of 1140 (“Cl) and 187 (37Cl) lines with standard deviations of 5.8 and 6.7 x lo-* cm-‘, respectively. 0 1992 Academic Press. Inc.

I.

INTRODUCTION

The infrared spectrum of natural CF3Cl has been extensively investigated in the last years under medium or high resolution, and the rovibrational analysis of selected bands led to the evaluation of accurate spectroscopic parameters and to the interpretation of different interaction mechanisms in perturbed states as well (I-5). Although heavy molecules, like CFsCl, have very small rotational constants of ca. 0.1 cm-’ magnitude, the availability of Fourier transform (FTIR) and diode laser spectrometers brings this molecule into the range where resolution of individual lines in the rovibrational spectra becomes possible. Overtones and combination bands are valuable sources for additional information on fundamentals, anharmonic constants, and resonances. Rotational details of the 2~: overtone were measured a few years ago in our laboratory using a tunable diode laser spectrometer and the excited state parameters have been determined for both isotopomers (6). As a part of a more complete investigation of the CF&l spectra in the g-pm region we have recently completed a study on the v2 + us combination and its hot bands (7). The present contribution extends the high-resolution analysis to the perpendicular band vz + vs near 1345 cm-‘. This combination has been previously analyzed for CF335C1under medium resolution by means of least-squares refinement and band contour simulation methods (8). By using the tunable diode laser spectrometer, more details have now been obtained and most of the individual lines have been resolved. Difficulties arising from overlap ping of hot band absorptions have been partly eliminated using cold spectra at about 240 K. This work deals with the measurement, the interpretation of the rotational structures, and the evaluation of the molecular constants of the u2 + u5band for both isotopic species of CFsCl. 265

0022.2852192 $5.00 Copyright 0

1992 by Academic Press, Inc.

All rights of reproduction in any form reserved.

266

GIORGIANNI II. EXPERIMENTAL

ET AL. DETAILS

High-resolution spectra of CF$l (purity 3 99%) with natural isotopic composition have been recorded in the range 1332- 1360 cm-’ using the tunable diode laser spectrometer at the University of Venice. In the present version the spectrometer setup is in a triple-beam configuration so that the sample spectrum, the calibration lines, and the reference signal from a solid Ge &talon are detected simultaneously. An IBM PC XT microcomputer interfaced to the instrument was employed for data acquisition, storage, and elaboration of the spectra. The measurements were performed with a 49-cm-long absorption cell at pressures ranging from 0.3 to 1.5 mbar and at low temperature (-240 K) in order to reduce the interferences arising from hot bands. Absolute calibration of the spectra was based on the wavenumbers of SO, lines measured on the Fourier spectrometer Bruker IFS 120 HR in Giessen against Hz0 lines (9); the relative calibration was obtained with a 2.59-cm germanium &talon having a fringe spacing of about 0.0475 cm-‘. The absolute wavenumber accuracy is estimated to be better than 0.0004 cm-‘, and the internal consistency within the same laser mode is of the order of 0.0002 cm-‘. III. RESULTS

AND DISCUSSION

Natural trifluorochloromethane mainly composed of two isotopic varieties CF335C1 (-75%) and CF337C1(-25%) is a prolate symmetric rotor of C3” symmetry. The v2 + v5 vibration of E species produces a perpendicular-type absorption near 1345 cm-’ with well-pronounced Q branches close to the band center. The density of the lines in the region investigated (1332- 1360 cm-‘) is expected to be considerable because of the presence of the two isotopes and several hot bands of substantial intensity. In order to simplify the spectrum, the measurements have been performed at low temperature, and most of the residual weak features from hot bands are hardly detectable since they are buried under the v2 + v5 lines of the main isotopomer. Description of the Spectrum and Its Interpretation The v2 + vs band is regularly shaped, and the prominent Ipa)QK branches exhibit under high resolution a red-degraded Jfine structure mainly depending on the positive c? value. As an example, Fig. 1 reproduces the displacement of the J lines in the “Q3 branch reaching the kl = 4 level. From a medium-resolution study on this band for the main species a value of about 103 X 10m6cm-’ for the (Y’ constant has been determined (8), and hence it would be impossible to resolve lines with low J values. Concerning the P and R branches, the strongest features corresponding to the ‘PK(J) and RRK(J) transitions appear as groups of closely spaced lines showing distinct patterns which in some cases can be easily assigned by considering the expected intensity alternation arising from nuclear spin statistics. Each cluster can be defined by a Jmax value, and the spectral structure is composed by a series of lines having the ground state quantum numbers (J, 0) (only for AK = +I transitions), (J-2, l), (J-4, 2), . . . , down to (J-2(K-l), K-l), (J-2K, K), where J = J,, and K is equal to J/3, (J-1)/3, or (J-2)/3. In particular, the line starting the series has the quantum numbers (J-2K, Ii) and the degradation proceeds toward lower wavenumber in ‘PK(J), or higher wavenumber sides in RRK(J) manifolds with transitions down to (J-2, 1) or (J, 0), respectively. Furthermore, lines with low K values are shifted from the expected position by the 1(2,2) rotational resonance (see below). As one moves away from the band origin,

u2 + q BAND OF CF,Cl

267

c

134613841

cm-’

134&032

FIG. 1. J-resolved fine structure of RQ3(J) cluster of CF3”CI v2 + Yeband. (a) SO2 calibration lines; (b) experimental spectrum (P = 1 mbar, T = 240 K); (c) Ge &Ion fringes.

the clusters begin to run into one another, the spectral structure becomes increasingly dense, and the regular pattern of absorptions is more difficult to identify. Inspection of the spectra in greater detail discloses striking differences in the P and R branches. While the ‘P clusters are characterized by a well-stretched degrading structure, in the RR systems, on the contrary, the features are very close and the structure becomes more and more compressed with increasing the Jmaxvalue. A typical example is given in Fig. 2, where two spectral portions around 1335 and 1356 cm-’ show the general appearance of the ‘PK(J) and RRK(J) manifolds, respectively; the latter section also exhibits a few very clear contributions of CF337Clwhose identification has been depicted. Comprehensibly, the analysis of the v2 + v5 band began with the main isotopic variety and the weak lines of CF337C1,mostly dispersed under the very strong absorptions of 35C1,required a tedious procedure until a reliable assignment was achieved. Referring to the more abundant species, features of the RRK(J) clusters reproduced in Fig. 2 acquire some prominence from the superposition of 37Cl lines in certain J intervals, and the good agreement between the experimental and the simulated spectrum, generated with the molecular constants given later, proves unambiguously that the proposed assignments are consistent with the observed characteristics.

268

GIORGIANNI

ET AL

-IK JK

pPx(Jmax=45)

pPK(J,ax=46) J=(J

IaX-2K)

I

I

1334.9558

cm-1

1335.2495

B a

b

9

12

15

16

15

‘%I JdJ

..X

-2K)

“Cl

’

I 6

I

1355.8676

12

0

I I IIll ’

’

I 9

’ IO

I I 1 12

15

K

16 I I

cm-1

1356:1638

FIG. 2. Details of the P and R branches of the CF,CI ~2 + ~5band (P = 1.3 mbar, TN 240 K) near 1335 and 1356 cm-‘, respectively. (A) Lines of ‘Z+(J) clusters of 35Clwith J,,,, = 45 and 46 are depicted. (B) Transitions of RR,@) clusters with J,, = 48,49 (35Cl)and 55,56 (“‘Cl) are indicated. (a) Simulated mm; (b) experimental spectrum. Lines marked with an asterisk in the 35C1clusters include several unresolved transitions with low K values.

v2 + v5 BAND OF CF&l

269

Interestingly, the degradation of the ‘P and RR clusters of CFj3’C1 shows a different trend with respect to what observed in the main isotopomer. In particular, the series always starts with a line having the ground state quantum numbers (J-2K, K), but the degradation proceeds toward higher wavenumbers in ‘PK(J), and lower wavenumbers in RR~(J) manifolds exhibiting a compressed and a stretched structure, respectively. Data Analysis The rotational analysis has been performed using a computer program for a perpendicular band of a symmetric top, which explicitly treats the 1(2,2) rotational resonance within a degenerate vibrational state. The ground state energy up to the quartic coefficients is given by E(J, K) = &J(J + 1) + (A0 - Bo)K2 - DyJ’(J + 1)’ - D$J(J + 1)K2 - D$K4. (1) The unperturbed is defined by

energy up to the fourth order for the E state (11~= o5 = 1, 1 = f 1)

E(u, J, k, r) = v. + &J(J + 1) + (A, - B,)k2 - D;J’(J + 1)’ - DyKJ(J + l)k2 - Dj;k4 - [2(Ar)” - $J(J + 1) - &ti]kl,

(2)

and the off-diagonal element for the 1(2,2) resonance within (v2 + z+)‘~ was taken as (V2 =

u5

=

= -f[&

l,l=

1, J,k+

11H1u2=u5=

l,l=-l,J,k-

1)

+ ~&J(J + I)][J(J + 1) - k(k + I)]‘/~[J(J + 1) - k(k - I)]*/~ (3)

with the sign convention of q& according to Ref. (10). The individual transitions, neglecting the 1(2,2) resonance, can be represented by suitable relations derived from the Eqs. (l)-(2) following the well-known selection rules. In spite of the discontinuity of the laser modes, the measured spectrum allowed us to identify many ‘$(P, Q, R) lines for CF335C1and a considerable number of CF337Cl features which, as known, are much weaker than those corresponding to the main isotopic variety. The analysis was carried out according to commonly adopted procedures. Preliminary line positions were computed employing for the ground state the constants Bo, D:, DyKfrom microwave measurements (I I), A0 from electron diffraction structure as quoted in (12) and references given therein, and 0% from harmonic force field (13), and for the upper state the parameters from Ref. (8). Concerning the 37C1 species, the band origin has been estimated from the value of the main isotopomer by considering the isotopic shifts of y2 from Ref. (14) and vg from the results of 2~2 band (6), while the d5, ~$5, and (A[325 constants, as a reasonable first approximation, have been fixed to the values of the “Cl variety. The pseudoband structure aided considerably in the line assignments, which were established by also taking into account relative intensity considerations, and the analysis proceeded iteratively with revision of constants and prediction of new transitions. The region investigated led to the identification of many lines in the P,R(P,Q, R) subbranches and the individual measured transitions covering the ranges -28 < KAK G 24 up to J = 68 (CF335C1),and - 18 < KAK G 24 up to J = 58 (CF337C1)are collected in Tables I and II, respectively, which besides the observed frequencies also include the (0 C) values and the adopted weights (see later). When lines with low K and high J values were included in the fit, it was necessary to consider the 1(2,2) resonance which links the ) v2 + v:‘, J, k + 1) and 1v2 + v;‘, J, k- 1) levels. This perturbation is significant even for other low K values and not only

270

GIORGIANNI

ET AL.

TABLE I Observed Line Positions (cm-‘) of the CFj3%1 v2 + v5 Band LINE

085

o-c* ”

LlNE

00s

O-C’*

L*HE

a O-C is (Observed - Calculated) in units of 10m4cm-‘.

085

O-C0”

UNE

085

0-c’ w

~2 + v5 BAND

OF CF,Cl

271

TABLE I-Continued

085

a-c’ w

LINE

085

0-P *

LINE

ms

0-c’ *

UNE

cm

0-c’ w

GIORGIANNI

ET AL.

TABLE I-C’onlinued

for the kl = 1 level, where the expected enhanced effect produces the classical AI/A2 splitting. This is evident from the irregularity of the pattern of the RQo lines leading to a substantial broadening of the cluster itself; in particular the observed transitions of the 35C1species are red-shifted as compared to the computed unperturbed positions, and the lines are pushed down about 0.0 17-O. 138 cm-’ on going from J” = 18 to 54.

273

vz + u5 BAND OF CF3Cl TABLE II Observed Line Positions (cm-‘) of the CFj3’C1 vz + v5 Band LLNE 085

Ll-caw

LlNE

085

0-P H

LINE

085

o-ca Y

LlNE

085

0-P w

This behavior, characteristic of transitions reaching the lower component of the split sublevels, is consistent with the positive sign of the q& constant. Therefore, the shift of the perturbed lines toward lower wavenumbers combined with the red-shaded J structure in the Q&J) branches gives a straightforward explanation of the broadening of the RQo cluster. Similarly, since between the interacting levels I v2 + u:‘, k + 1) and 1u2 + v;l, k - 1) the former are higher in energy than the latter, the perturbed ‘QK and RQKmanifolds appear slightly broadened and narrowed, respectively, as compared to the unperturbed features. Instead, in the 37C1species most of the transitions are heavily masked by the much stronger lines of the main isotopomer, and hence the interaction effect on the shape of the QK(J) clusters is not clearly visible in the spectra. All the assigned transitions were employed to determine the spectroscopic parameters. Unit weight was given to lines which from their intensity and width appeared as single transitions, while a lower weight (0.1) was chosen for the blended or scarcely resolved features. A total of 1140 (CF335C1)and 187 (CF337C1)lines were correlated in the final fit (a 2: 6 X 10m4cm-‘) and the derived molecular constants for both isotopomers are given in Table III. In the refinement process the ground state parameters have been fixed and only the band origin and the excited state constants were allowed to change. When the fit of 35C1lines did not include the d5 constant, small differences between observed and calculated wavenumbers were observed for transitions with low K and very high J values. Discrepancies have been removed by refining the same data with d5 as addi-

274

GIORGIANNI

ET AL.

TABLE III Molecular Constants (cm-‘) for the q + us Band of CF,CP

Ao 60 D;

x

% D;

IO8 x

10

x

8

108

Vo (A~-A~)

Y

0.1913b

0.1913b

0.11126346’

0.10B46101c

1 .Ei44c

1.759C

6.93’

6.72’

-4.12b

-3.85b

1344.86645(S)

1343.43837(18)

10~

3.144(4)

3.218(12)

(EO-B')

x

10 4

CO;-D;)

Y

10 9

-0.15(l)

0.15(8)

x

-0.18(13)

2.15(64)

(D&-D~~) CO;-D;o

x

0.9446(6)

10 9

10 9

1.30(69)

'IJ x

IO

‘IK

10

x x

J 425

x

NO.

of

lJ x

10

a

0

-0.137473(2)

(MI25

G5

0.9306(26)

8 7

3.7(2)

3.6(7)

-5.02(9)

-4.26(39)

10 4 10

0.928(3)

9

0.887(6)

0.4(l)

data

0

1140

3

Quoted

-0.137970(6)

187 0.583

uncertainties

the last

0.668

are one standard

significant

deviation

in units

of

digit.

b

Fixed

to

the

values

'

Fixed

to

the

microwave

given

in

values

Ref.(12). of

Ref.(ll).

tional free parameter. Of course, the latter constant has been also tested by being included in the final fit of 37C1transitions, with no improvement in the results; since the refinement of this constant as well as of (0% - I&) were physically not meaningful they were fixed to zero. The obtained parameters, with the exception of (0% - Dk) and q& for “Cl, are all apparently well determined, and since the rotational and centrifugal distortion terms are very close to their ground state values, perturbations of v2 + u5 with other vibrational levels are expected to be of little significance. To show how well the obtained spectral parameters reproduce the observed features, a small portion of the simulated spectrum near 1350 cm-’ is compared to the experimental one in Fig. 3, which mainly involves the RQlQ) manifold and the RRx(J) clusters (JmaX= 2 1, 22) of the main species, and few weak characteristics arising from 37C1variety. As can be seen, the agreement between the observed and synthetic spectra is quite good, thus pointing out the reliability of the determined molecular constants.

~2 + ~5 BAND OF CF$ZI

I I I

I

I I I

55

I

I

I I I I

50

I

I

I 11 1

45

40

I I I I

275

III~IllldIIlI~ 35

30

25

20

J

%,,(J) I 1350.1088

I cm -1

1350.3200

FIG. 3. Part of the Y*+ Yeband of CF&l near 1350 cm-‘. J-resolved fine structure of ‘Q,*(J) manifold and details of the RR,&) clusters (J,,,= = 21 and 22) of CFa”Cl are labeled. (a) Simulated spectrum: (b)

experimental spectrum (P = 1 mbar, T = 240 K).

In conclusion, from this investigation many lines with high J and K values of the v2 + v5band have been identified, and the measurements yielded sufficient information to obtain very accurate values of molecular constants up to the fourth order for both isotopomers. The present data are in agreement with those available for 35C1from Ref. (a), but are much more complete and more accurately determined. The effects of the Al = Ak = k2 resonance are not restricted to the kl = 1 level, and appreciable shifts have also been found for other levels with low K and high J values. The 37C1 parameters are slightly less precise than those for the main species, and this seems justified by the lower quantity of transitions employed in the fit. However, the standard deviations for the observed line positions are comparable to the precision of the data, and this shows that the adopted model is satisfactory and no additional parameters are required.

GIORGIANNI

276

ET AL.

ACKNOWLEDGMENTS

We express our gratitude to Professors M. Winnevisser and A. Gambi for making Bruker SO2 calibration spectra available to us. We thank Mr. M. Pedrali for operating the diode laser spectrometer. Financial support by MURST, Roma, through 40 and 60% programs is gratefully acknowledged. RECEIVED:

December 17, 199 1 REFERENCES

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Acta Part A 39,985-992 (1983). 9. G. GUELACHVILIAND K. NARAHARIRAO, “Handbook of Infrared Standards,” Academic Press, New

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10. 11. 12. 13.