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Diode laser i.r. spectrum and rovibrational analysis of CF3Br in the ν2 + ν3 region
Specrrochimico Acta, Vol. 4SA. No. 3, pp. 329-334. Printed in Great Britain.
1989. 0
05848539/89 s3.co+o.lm 1989 Pergamon Press pk
Diode laser ix. spectrum and rovibrational analysis of CF,Br in the v2 + vj region A. BALDACCI, S. GIORGIANNI, Dipartimento
di Chimica
Fisica, Universitl (Received
P. STOPPA and S. GHERSETTI di Venezia, D.D. 2137, I-30123 Venezia, Italy
1 August
1988)
Abstract-The gas phase i.r. spectrum of CF,Br, with natural isotopic abundance, has been investigated in the vt + vJ region near 1120 cm- ’ using a tunable diode laser spectrometer. The measurements have been carried out at low temperature (~200 K) to minimize the effects due to the “hot” band absorptions. The K structure of many P(J)and R(J) manifolds has been resolved and analyzed: the maximum J value reached for individual lines was 62 and 70 for CF, 79Br and CF, *lBr , respectively. The identified transitions have been used in a least-squares fit to the energy expression up to the quartic terms and molecular parameters for the v2 + v3 combination have been obtained. Residual weak features due to “hot” bands of vJ and vs have been assigned; the J structure has been analyzed by means of a polynomial procedure and spectroscopic constants for both the isotopomers have been derived.
INTRODUCTION
Numerous studies have been devoted in the last few years to the gas phase i.r. spectra of CF,Br in order to understand the structure of the rovibrational absorptions and to yield spectroscopic parameters of the fundamental, overtone and combination bands [l-S]. The availability of a tunable diode laser spectrometer in our laboratory, working around 9 pm, allows us to obtain rotational details in the gas phase spectra of heavy molecules having rotational constants of ca 0.1 cm- 1 magnitude or less. On this basis, diode laser spectra of the CF,Br v1 fundamental have very recently been subjected to detailed studies, and accurate constants for both the isotopic species have been determined through analysis of the resolved K structure [6]. As a part of a more complete investigation in this spectral range, the recording of the diode laser spectra has been now extended to the v2 + vj band lying near 1120 cm-‘. For this vibration, the Fermi resonance with v1 and 2~: levels is certainly not the only occurring perturbation and further interactions with the nearby state 2~: 2 can be also expected. Previous studies on the v2 + vj combination were carried out on medium resolution spectra several years ago but at that time no resolved K structure for the P(J) and R(J) multiplets could be detected [S]. The present work deals with the observation, measurement and interpretation of the v2 + v3 fine structure and the evaluation of the related molecular constants for CF 3 79Br and CF,*lBr. Results from the analysis of “hot” bands of v3 and vg for both the isotopic modifications are also reported.
Inc. Two diode lasers supplied by the same company covered the whole region examined except for small gaps in frequency, where no laser modes were available. The measurements were carried out at a pressure ranging from 0.5 to 1.5 Torr at about 200 K usine a home-made 12 cm long stainless steel cell cooled by-flowing liquid nitrogen vapour. The cold spectra are extremely useful for molecules with low frequency modes, as is CF,Br, in order to reduce the strong interference from “hot” band absorptions and make reliable assignments. Absolute wavenumber calibration was performed by means of well known NH, and N,O lines [7’j and suitable SO, transitions, the frequencies of which were determined using the constants of Ref. [S]. Relative wavenumber calibration was performed with a 2.59 cm germanium &talon with a fringe spacing of about 0.0476cm-‘. The absolute wavenumber accuracy, also Iimited by the secondary standards, is estimated to be around 0.003 cm- ‘, while the internal consistency within the same laser mode is of the order of 0.001 cm-‘.
ANALYSIS AND RESULTS
Natural trifluorobromomethane is mainly composed of two isotopic varieties (CF,79Br and CF, * ‘Br) in almost equal proportion. This molecule, belonging to the C,, point group, is a prolate symmetric rotor. Although the measurements were performed at low temperature (N 200 K) the rovibrational system v2 + vg is still accompanied by weak features coming from the “hot” bands v2 +2v, -v3 and v,+v,+v,-v,. Information on the number of the observed lines of each band together with the maximum value of J and K reached for both the isotopic species are collected in Table 1. The v2 + vg band
EXPERIMENTAL High resolution spectra of have been recorded in the range the diode laser spectrometer mainly based on the SP-5000
natural CF,Br (99% purity) 1108-l 128 cm-‘, employing at the University of Venice, assembly of Laser Analytics
The v2 + v3 combination of A, species produces a parallel-type absorption. Unperturbed parallel bands should have their K-component structure in a regular order since line positions within a multiplet mainly depend upon (AA-AB)K2. 329
A.
330 Table
1. Data concerning
Isotopic form
BALDACCI
the bands analyzed
J:,,
Band
P
CF 379Br
“,+“a v,+2v,-v, v,+v,+v;‘-v,$’
CF 3 *rBr
“Z+“, v,+2v,-v, v,+v,+v;‘-v;’
62 75 65 66 71 57
Y
R I68
R(67I
iR( 501
CF”Br 3
et al.
R 55 47 61 70 63 70
in the v2 + vj region of CF,Br
Gl,x 38
45
Number of data 246 57 44 326 56 50
SD x 104cm-’ 8 8 I 8 6 8
1 R(49)
112617703
et-4
I 1126 5238
Fig. 1. Portion of the R branch spectrum (P- 1.5 Torr) at different temperatures of the CF,Br v2 + v3 band. (a) Spectrum recorded at ~298 K. (b) Spectrum recorded at -200 K. Upper trace: Laser mode signal showing N,O absorption lines. Lower trace: Germanium &talon fringes.
From constant
a medium of about
resolution -44
study
x 10m6 cm-
[S] a (AA - AB)
’ has been dedu-
ced for this band, i.e. a value much smaller than that, for example, of - 185 x 10m6 cm-’ determined for the vr fundamental [6]. On this basis, the density of the lines in the P(J) and R(J) multiplets is expected to be considerable and the fine structure should be resolved only for high values of J and K. Moreover, the spacing
between the J manifolds, roughly corresponding to 2B~0.14 cm-‘, should be large enough to avoid superimposition of rotational details of the two contiguous clusters coming from the same isotopic variety. A typical example of the spectrum examined is given in Fig. 1, which reproduces a small portion of the R branch, exhibiting clusters of the different isotopomers
331
The vI + vj band of CF,Br (the marked degradation to lower frequency within each multiplet is, of course, mainly related to the negative value of (AA -A@). In the upper part of the figure the room temperature spectrum is also included; this is only to show how the rotational structure becomes confused when “hot” band transitions of considerable intensity are present. At low temperature the rotational details appear sufficiently clear to enable unambiguous assignments. Within each multiplet the K structure appears resolved for K > 15 and the separate lines show the expected intensity alternation. Due to spin statistics considerations, transitions between levels with K = 3n (including K=O) are twice as strong as those with K + 3n. The distribution of the K components in both wavenumber and intensity is depicted in Fig. 2 where a small portion of the P branch near 1111 cm-’ is reported. Besides the strong P(J, K) clusters coming from the v,+v, band, the spectrum also shows a number of weak features, which have been associated with “hot” bands, as will be seen later. The individual transitions can be expressed by suitable relations derived from the equation reported below. The energy of the rovibrational levels, including the quartic centrifugal distortion coefficients, is
h
given by: E(o, J, K) = E, + (A, -
B,)K2+ B,J(J + 1)
-D;l.fz(J+1)2-D;,J(J+ 1)K2-DkK4. (1) An interesting feature deals with the K displacement behaiiour within the multiplets; the fine structure becomes more compressed as J increases and such a characteristic is more marked in the P(J) than the R(J)manifolds. This can be understood if, in the equations for the transition frequencies, the K-dependent part is considered. The terms responsible for the position of any line relatively to the K =0 transition within each cluster are expressed as follows: v(P(J,
K)--(.I, O))=[(AA-AL?) + 250;‘, -J(J
- 1)AD,,]K2
-AD,K4 v(R(J, K)-R(J,
(2)
O))=[(AA-AB)-2(J+ -_(J+ -
l)D;‘,
1)(J+2)ADJK]K2
AD,K4.
(3)
is
.iO
. 3-o
24
HG
24
hl
F HG
F
.18
‘18
/
PM1
PI621
CF;Br
PI501
PI481
I
I 1110.9683
CF %r 3
cm'
1110 6862
Fig. 2. Detail of the P branch spectrum (T2200 K, PZZ 1 Torr) of the CF,Br v2 + vj band near 1111 cm- ‘. K assignments of P 6142 (CF, 79Br) and P 48-50(CFSa1Br) are indicated. F=v, +2v,-v,, ‘IBr (P 55-57 from left). G = v2 + v3+ v6- v6, 79Br (P52-53). H = v2+ vj+ v6- v6,slBr (P 3940).
332
A. BALDACCI et al.
For high values of J( N SO), the contribution of 2JD;‘, and -2(J+ l)D;‘, (D&=4x 10-s cm-‘, Ref. [9]) starts to become significant and because of the different signs of (A,4-AB) and D’&, the KZ term will be smaller for the P branch than for the R branch transitions. Furthermore, from the above equations it can be deduced that within each multiplet the K structure will be resolved for K > 15. Many P(J) and R(J) clusters of both the isotopic species were analyzed and most of the lines were resolved and assigned. The rotational analysis was carried out following standard techniques and a computer program, which performs a least-squares fit of the observed lines to produce a set of molecular constants, was employed. Preliminary line positions were calculated using for the ground and the upper state the parameters from Refs [9] and [S], respectively. The identified transitions employed in the leastsquares fit yielded improved molecular constants, which were used in turn to compute more accurate frequencies, and further assignments were performed. As an example, Table 2 lists identification and position of the lines involved in the CF,*lBr P(49) multiplet, already shown in Fig. 2. The excellent agreement between the observed and calculated frequencies points out the quality of the fit to the observed data. A total of 246 (CF,79Br) and 326 (CFJ8’Br) P, R(J, K) transitions were correlated in the final least-squares fit (g N 8 x 10m4 cm- ‘) and the spectroscopic constants for v2 + v,*of both the isotopic varieties are given in Table 3. The v2 + 2v, - vj and v2 + vj + v6 - v6 “hot” bands The rovibrational system v2 + v3 of both isotopomers is accompanied by “hot” bands, the intensity of which is still considerable even at low temperature. The strongest identified features belong to vZ+2v, -vJ and v,+v, +v,-v, bands, having at 200 K a relative intensity to v2 +v, of about 16 and 22%, respectively. For the “hot” band of vg the perpendicular component is reasonably supposed to be quite weak and therefore the present analysis deals with the
Table 3. Molecular constants (cm- ‘) for the v2 + Ye band of CF,Br* CF 3 79Br ;;’ B” (.4-A”) X 104 D; x lo9 D” x lo9 D!, X 10s D” x 108 (6:-D;;) x lo9
1120.0529 (3) 0.0698267 (3) 0.06998597t - 2.033 (6) 8.17(8) 8.99t 4.06 (2) 4.34.t 2.41
J
K
Observed
O-C
49
15 18 21 24 27 28 29 30 31 32 33 34 35 36
1110.8402
0.0010 0.0010 0.0006 0.0006 0.0006 0.0008 o.OOQ7 0.0002 0.0007 0.0002 0.0007 0.0002 0.0005 0.0@03
1110.8367 1110.8322
1110.8274 1110.8218 1110.8200 1110.8177 1110.8150 1110.8132 1110.8103 1110.8083 1110.8052 1110.8028 I1 10.7998
1117.9996 (2) 0.0691795(2) 0.0693334tt - 1.980(5) 8.91 (3) 8.87t 3.99(l) 4.27t 2.4 (2)
*Numbers in parentheses are standard deviations of the last digit quoted. tFixed to the microwave results of Ref. [9], SConstrained to the value of CFa8’Br.
in units
A, type component,
for which we expect a structure very similar to that of an ordinary parallel band. In addition, it should also display a splitting of the lines increasing with K, which offers a direct measure of the change in (A& between the degenerate levels. The observed features being weak and narrow, the rotational analysis has been limited to the J peaks only, even if in some cases a poorly resolved K structure is exhibited by the clusters examined. In spite of the discontinuity of the laser modes, a relevant number of P(J) and R(J) clusters of “hot” bands was measured. The identification as assisted by computed frequencies according to the parameters of Refs [3, 53 was checked, when possible, by the usual lower state combination differences, from the data concerning the v3 = 1 and vg = 1 states available in Ref. [9]. The rotational analysis was carried out using the polynomial method and the J peaks were fitted to the well known equation: vP.R=A+Bm+Cm2+Dm3,
(4)
with A=v,+[(A’-A”)-(B’-B”)]K’ B=B’+B” C=B’-B”-(D;-D;‘)?
Table 2. Line positions (cm- ‘) of the P(49, K) multiplet of the CF,“Br Y*+ v3 band
CF 3 *‘Br
-as
D= -2(D;+D;‘). The polynomial was expanded, successively introducing higher powers of m without improving the fit; the quartic term Em4 could not be determined significantly. If the additivity of tlAvB is assumed and the maximum of the J peaks is taken for K values not higher than 3, thequantity [(A’-A”)-(B’-B”)]K’ is very small (< -4 x 10m4 cm- ‘). Therefore, no correction for K structure of the frequencies appeared suitable. The molecular constants for the v2 +2v,-v, and “hot” bands of both isotopomers, V2+V3+V,--Vfj derived from the coefficients of the polynomial, are set out in Table 4 which also reports, for the sake of comparison, the B values of v3 and vg available
The vr + vj band of CF,Br
333
Table 4. Molecular constants (cm- ‘) for the vr + 2v, - vs and v2 + vj + vg- vg “hot” bands, and a”. Bvalues obtained in the v2+ vj region of CF,Br CFa8iBr
CF 379Br v,+2v,-v,* ; B” (D$+D;‘)x IO9
v,+vs+v~-vvs*
1120.9422 0.069776 (6) (9) 0.069944 (9) 0.0699346t 22 (4)
lo6 a;3 x lo6 afS3 x lo6 a$!s6x lo6 a! x lo6
v,+2v,-v,*
v,+v,+v,-v,*
1119.1293(4) 0.069129 (7) 0.069296 (7)
1116.5184(6) 0.069041(10) 0.069198(10) 0.0692116t 18(4)
1118.5593(6) 0.069710(10) 0.069867(10) 0.0698636t 25 (3)
198.0(S) 153.9(2) 204 (7) 292(10) 37(7)
203.3 (6) 159.3(3) 210(9) 276(10) 42 (9) 51.4t 119(10) 122.4.t
a$, x
a: x lo6
23 (3)
135(10) 121.8t
*Numbers in parentheses indicate the uncertainties (3 x SD). t From the microwave results of Ref. [9].
from microwave measurements [9]. For completeness, this table also includes the tlAsB coefficients derived from the bands analyzed in the present work. DISCUSSION
The complete analysis of the vs +v, region of CF,Br allowed us to identify more than 550 individual transitions for the main band and a considerable number of J peaks for the “hot” bands v1 + 2v, - vj and v2 + vj +v,-v, of both the isotopic modifications. The observed line positions, together with the assignments and the (O-C) values for all the bands investigated, are compiled in Tables 5-7, which are available as supplementary material.* In the refinement procedure of the v,+v, parameters (see Table 3), the ground state constants (B”, 0;’ and D;Ik) were fixed to the microwave values [9], and only the upper state constants and the band origin were allowed to change. The small quantity (Ok --Di) was significantly determined only for the CF,*lBr species and the obtained value was used, as a constrained constant, in the final least-squares fit of the CF,79Br data. The sextic coefficients were also tested without improving the results. Figure 3 shows a small portion of the observed and simulated spectrum near 1127 cm- ‘. As can be seen, the experimental features match quite well with the simulated ones even for the “hot” band characteristics, and this accounts for the good quality of the results achieved. As expected, the envelopes of the J clusters in the “hot” bands were found to be sensitive to the us *Tables 5-7 have been deposited with the British Library, Boston Spa, Wetherby, West Yorkshire, U.K., as supplementary publication No. 13009 (4 pp.).
-c8 value. While U* was well determined from the polynomial analysis, attempts to extract K“ by computing a series of J manifold contours were not successful. Since the J peaks are weak and narrow, not so precise values of a” could be obtained and therefore it was decided to fix this parameter to the corresponding value for the v2 + vJ band; such an approximation is legitimate if the additivity of aATE is assumed. In addition, in the simulation of v2 + vj + vi ’ - v,i ’ features, (A[)’ was supposed to be equal to (A[)” and the reproduction obtained was deemed sufficient for our present use. This procedure leads us to neglect the splitting of K transitions into + 1and - 1components, which would be small when the same degenerate vibration is involved in the lower and upper state and no perturbation occurs. As mentioned above, interactions of the v1 + vj level with the nearby states are also expected. The Fermi resonance with vi and 2~: is, however, global and it is accounted for by the effective J and K dependence of the rotational constants. Following the results on CF,Cl [lo], from the value of 2~: of CF,Br [l l] it is possible to estimate the band origin of 2vt2 (F 1095 cm-‘), if the anharmonicity contribution is assumed invariant in the CF,X series. On this basis, considering the reduced energy levels of v,+v, and 2v*’ 5 Ta AI=Ak= +2 type interaction is expected to occur causing a crossing around K = 23 in 2~: 2. At this stage, no appreciable rotational resonance effects have been detected for transitions with K =21 in the v2 + vs band. In conclusion the present investigation allowed us to obtain a well conditioned set of spectroscopic parameters for the v2 +v, combination and related “hot” bands. The band origin and the rotational constants Q” and us are in satisfactory agreement with
A. BALDACCI et al.
334
GF
a
R171)
RI701
CF_"Br
:
I(
R1541
RI531
RI521
cm’
1127 0074
I li27 2530
I
Fig. 3. Detail of the R branch spectrum of the CF,Br Ye +v, band near 1127 cm-‘. (a) Experimental spectrum (T1200 K, P2 1 Torr). (b) Simulated spectrum including “hot” bands. K structure of R 54-52 of CF,79Br is labelled. E = vZ + 2v, - vj, 79Br (R 4745 from left). F = v2 + 2v, - vl, “Br (R 63-60). G = v1 + Ye 79Br (R 67-64). +v,--v,,
those available from Refs [4, 51, but are more accurately determined; a good additivity in the aB values (see Table 4) was also found. The quartic coefficients for the v2 + v3 band of both isotopomers were determined for the first time.
[4]
H. BURGER, K. BURCZYK, P. SCHULZ and A. RUOFF, Spectrochim. Acta 38A, 627 (1982).
[S]
K. BURCZYK, H. BURGER, A. RUOFF and M. MORILLON-CHAPEY, Spectrochim. Acta 37A, 615 (1981). A. BALDACCI, S. GIORGIANNI, R. VISINONI and S. GHERSETTI, J. molec. Spectrosc, in press. G. GUELACHVILY and K. NARAHARI RAO, Handbook of Infrared Standards. Academic Press, New York (1986). J. P. SATTLER, T. L. WORCHESKY and W. L. LAFFERTY, J. molec. Spectrosc. 88, 364 (1981). J. H. CARPENTER, J. D. MUSE and J. G. SMITH, J. them. Sot. Faraday Trans. II 78, 139 (1982). S. GIORGIANNI, A. BALDACCI, R. VISINONI and S. GHERSETTI, J. molec. Spectrosc. 130, 783 (1988). H. BURGER, K. BURCZYK, H. HOLLENSTEIN and M. QUACK, MoIec. Phys. 55, 255 (1985).
[6] [7]
REFERENCES [I]
H. JONES, F. KOHLER and H. D. RUDOLPH, .I. molec. Spectrosc.63, 205 (1976). [2] K. BURCZYK, H. BURGER, A. RtJoFFand P. PINSON, .I. molec. Spectrosc. 77, 109 (1979). [3] A. BALDACCI, A. PASSERINIand S. CHERSETTI, J. molec. Spectrosc. 91, 103 (1982).
[8] [9] [lo] [ll]
1989. 0
05848539/89 s3.co+o.lm 1989 Pergamon Press pk
Diode laser ix. spectrum and rovibrational analysis of CF,Br in the v2 + vj region A. BALDACCI, S. GIORGIANNI, Dipartimento
di Chimica
Fisica, Universitl (Received
P. STOPPA and S. GHERSETTI di Venezia, D.D. 2137, I-30123 Venezia, Italy
1 August
1988)
Abstract-The gas phase i.r. spectrum of CF,Br, with natural isotopic abundance, has been investigated in the vt + vJ region near 1120 cm- ’ using a tunable diode laser spectrometer. The measurements have been carried out at low temperature (~200 K) to minimize the effects due to the “hot” band absorptions. The K structure of many P(J)and R(J) manifolds has been resolved and analyzed: the maximum J value reached for individual lines was 62 and 70 for CF, 79Br and CF, *lBr , respectively. The identified transitions have been used in a least-squares fit to the energy expression up to the quartic terms and molecular parameters for the v2 + v3 combination have been obtained. Residual weak features due to “hot” bands of vJ and vs have been assigned; the J structure has been analyzed by means of a polynomial procedure and spectroscopic constants for both the isotopomers have been derived.
INTRODUCTION
Numerous studies have been devoted in the last few years to the gas phase i.r. spectra of CF,Br in order to understand the structure of the rovibrational absorptions and to yield spectroscopic parameters of the fundamental, overtone and combination bands [l-S]. The availability of a tunable diode laser spectrometer in our laboratory, working around 9 pm, allows us to obtain rotational details in the gas phase spectra of heavy molecules having rotational constants of ca 0.1 cm- 1 magnitude or less. On this basis, diode laser spectra of the CF,Br v1 fundamental have very recently been subjected to detailed studies, and accurate constants for both the isotopic species have been determined through analysis of the resolved K structure [6]. As a part of a more complete investigation in this spectral range, the recording of the diode laser spectra has been now extended to the v2 + vj band lying near 1120 cm-‘. For this vibration, the Fermi resonance with v1 and 2~: levels is certainly not the only occurring perturbation and further interactions with the nearby state 2~: 2 can be also expected. Previous studies on the v2 + vj combination were carried out on medium resolution spectra several years ago but at that time no resolved K structure for the P(J) and R(J) multiplets could be detected [S]. The present work deals with the observation, measurement and interpretation of the v2 + v3 fine structure and the evaluation of the related molecular constants for CF 3 79Br and CF,*lBr. Results from the analysis of “hot” bands of v3 and vg for both the isotopic modifications are also reported.
Inc. Two diode lasers supplied by the same company covered the whole region examined except for small gaps in frequency, where no laser modes were available. The measurements were carried out at a pressure ranging from 0.5 to 1.5 Torr at about 200 K usine a home-made 12 cm long stainless steel cell cooled by-flowing liquid nitrogen vapour. The cold spectra are extremely useful for molecules with low frequency modes, as is CF,Br, in order to reduce the strong interference from “hot” band absorptions and make reliable assignments. Absolute wavenumber calibration was performed by means of well known NH, and N,O lines [7’j and suitable SO, transitions, the frequencies of which were determined using the constants of Ref. [S]. Relative wavenumber calibration was performed with a 2.59 cm germanium &talon with a fringe spacing of about 0.0476cm-‘. The absolute wavenumber accuracy, also Iimited by the secondary standards, is estimated to be around 0.003 cm- ‘, while the internal consistency within the same laser mode is of the order of 0.001 cm-‘.
ANALYSIS AND RESULTS
Natural trifluorobromomethane is mainly composed of two isotopic varieties (CF,79Br and CF, * ‘Br) in almost equal proportion. This molecule, belonging to the C,, point group, is a prolate symmetric rotor. Although the measurements were performed at low temperature (N 200 K) the rovibrational system v2 + vg is still accompanied by weak features coming from the “hot” bands v2 +2v, -v3 and v,+v,+v,-v,. Information on the number of the observed lines of each band together with the maximum value of J and K reached for both the isotopic species are collected in Table 1. The v2 + vg band
EXPERIMENTAL High resolution spectra of have been recorded in the range the diode laser spectrometer mainly based on the SP-5000
natural CF,Br (99% purity) 1108-l 128 cm-‘, employing at the University of Venice, assembly of Laser Analytics
The v2 + v3 combination of A, species produces a parallel-type absorption. Unperturbed parallel bands should have their K-component structure in a regular order since line positions within a multiplet mainly depend upon (AA-AB)K2. 329
A.
330 Table
1. Data concerning
Isotopic form
BALDACCI
the bands analyzed
J:,,
Band
P
CF 379Br
“,+“a v,+2v,-v, v,+v,+v;‘-v,$’
CF 3 *rBr
“Z+“, v,+2v,-v, v,+v,+v;‘-v;’
62 75 65 66 71 57
Y
R I68
R(67I
iR( 501
CF”Br 3
et al.
R 55 47 61 70 63 70
in the v2 + vj region of CF,Br
Gl,x 38
45
Number of data 246 57 44 326 56 50
SD x 104cm-’ 8 8 I 8 6 8
1 R(49)
112617703
et-4
I 1126 5238
Fig. 1. Portion of the R branch spectrum (P- 1.5 Torr) at different temperatures of the CF,Br v2 + v3 band. (a) Spectrum recorded at ~298 K. (b) Spectrum recorded at -200 K. Upper trace: Laser mode signal showing N,O absorption lines. Lower trace: Germanium &talon fringes.
From constant
a medium of about
resolution -44
study
x 10m6 cm-
[S] a (AA - AB)
’ has been dedu-
ced for this band, i.e. a value much smaller than that, for example, of - 185 x 10m6 cm-’ determined for the vr fundamental [6]. On this basis, the density of the lines in the P(J) and R(J) multiplets is expected to be considerable and the fine structure should be resolved only for high values of J and K. Moreover, the spacing
between the J manifolds, roughly corresponding to 2B~0.14 cm-‘, should be large enough to avoid superimposition of rotational details of the two contiguous clusters coming from the same isotopic variety. A typical example of the spectrum examined is given in Fig. 1, which reproduces a small portion of the R branch, exhibiting clusters of the different isotopomers
331
The vI + vj band of CF,Br (the marked degradation to lower frequency within each multiplet is, of course, mainly related to the negative value of (AA -A@). In the upper part of the figure the room temperature spectrum is also included; this is only to show how the rotational structure becomes confused when “hot” band transitions of considerable intensity are present. At low temperature the rotational details appear sufficiently clear to enable unambiguous assignments. Within each multiplet the K structure appears resolved for K > 15 and the separate lines show the expected intensity alternation. Due to spin statistics considerations, transitions between levels with K = 3n (including K=O) are twice as strong as those with K + 3n. The distribution of the K components in both wavenumber and intensity is depicted in Fig. 2 where a small portion of the P branch near 1111 cm-’ is reported. Besides the strong P(J, K) clusters coming from the v,+v, band, the spectrum also shows a number of weak features, which have been associated with “hot” bands, as will be seen later. The individual transitions can be expressed by suitable relations derived from the equation reported below. The energy of the rovibrational levels, including the quartic centrifugal distortion coefficients, is
h
given by: E(o, J, K) = E, + (A, -
B,)K2+ B,J(J + 1)
-D;l.fz(J+1)2-D;,J(J+ 1)K2-DkK4. (1) An interesting feature deals with the K displacement behaiiour within the multiplets; the fine structure becomes more compressed as J increases and such a characteristic is more marked in the P(J) than the R(J)manifolds. This can be understood if, in the equations for the transition frequencies, the K-dependent part is considered. The terms responsible for the position of any line relatively to the K =0 transition within each cluster are expressed as follows: v(P(J,
K)--(.I, O))=[(AA-AL?) + 250;‘, -J(J
- 1)AD,,]K2
-AD,K4 v(R(J, K)-R(J,
(2)
O))=[(AA-AB)-2(J+ -_(J+ -
l)D;‘,
1)(J+2)ADJK]K2
AD,K4.
(3)
is
.iO
. 3-o
24
HG
24
hl
F HG
F
.18
‘18
/
PM1
PI621
CF;Br
PI501
PI481
I
I 1110.9683
CF %r 3
cm'
1110 6862
Fig. 2. Detail of the P branch spectrum (T2200 K, PZZ 1 Torr) of the CF,Br v2 + vj band near 1111 cm- ‘. K assignments of P 6142 (CF, 79Br) and P 48-50(CFSa1Br) are indicated. F=v, +2v,-v,, ‘IBr (P 55-57 from left). G = v2 + v3+ v6- v6, 79Br (P52-53). H = v2+ vj+ v6- v6,slBr (P 3940).
332
A. BALDACCI et al.
For high values of J( N SO), the contribution of 2JD;‘, and -2(J+ l)D;‘, (D&=4x 10-s cm-‘, Ref. [9]) starts to become significant and because of the different signs of (A,4-AB) and D’&, the KZ term will be smaller for the P branch than for the R branch transitions. Furthermore, from the above equations it can be deduced that within each multiplet the K structure will be resolved for K > 15. Many P(J) and R(J) clusters of both the isotopic species were analyzed and most of the lines were resolved and assigned. The rotational analysis was carried out following standard techniques and a computer program, which performs a least-squares fit of the observed lines to produce a set of molecular constants, was employed. Preliminary line positions were calculated using for the ground and the upper state the parameters from Refs [9] and [S], respectively. The identified transitions employed in the leastsquares fit yielded improved molecular constants, which were used in turn to compute more accurate frequencies, and further assignments were performed. As an example, Table 2 lists identification and position of the lines involved in the CF,*lBr P(49) multiplet, already shown in Fig. 2. The excellent agreement between the observed and calculated frequencies points out the quality of the fit to the observed data. A total of 246 (CF,79Br) and 326 (CFJ8’Br) P, R(J, K) transitions were correlated in the final least-squares fit (g N 8 x 10m4 cm- ‘) and the spectroscopic constants for v2 + v,*of both the isotopic varieties are given in Table 3. The v2 + 2v, - vj and v2 + vj + v6 - v6 “hot” bands The rovibrational system v2 + v3 of both isotopomers is accompanied by “hot” bands, the intensity of which is still considerable even at low temperature. The strongest identified features belong to vZ+2v, -vJ and v,+v, +v,-v, bands, having at 200 K a relative intensity to v2 +v, of about 16 and 22%, respectively. For the “hot” band of vg the perpendicular component is reasonably supposed to be quite weak and therefore the present analysis deals with the
Table 3. Molecular constants (cm- ‘) for the v2 + Ye band of CF,Br* CF 3 79Br ;;’ B” (.4-A”) X 104 D; x lo9 D” x lo9 D!, X 10s D” x 108 (6:-D;;) x lo9
1120.0529 (3) 0.0698267 (3) 0.06998597t - 2.033 (6) 8.17(8) 8.99t 4.06 (2) 4.34.t 2.41
J
K
Observed
O-C
49
15 18 21 24 27 28 29 30 31 32 33 34 35 36
1110.8402
0.0010 0.0010 0.0006 0.0006 0.0006 0.0008 o.OOQ7 0.0002 0.0007 0.0002 0.0007 0.0002 0.0005 0.0@03
1110.8367 1110.8322
1110.8274 1110.8218 1110.8200 1110.8177 1110.8150 1110.8132 1110.8103 1110.8083 1110.8052 1110.8028 I1 10.7998
1117.9996 (2) 0.0691795(2) 0.0693334tt - 1.980(5) 8.91 (3) 8.87t 3.99(l) 4.27t 2.4 (2)
*Numbers in parentheses are standard deviations of the last digit quoted. tFixed to the microwave results of Ref. [9], SConstrained to the value of CFa8’Br.
in units
A, type component,
for which we expect a structure very similar to that of an ordinary parallel band. In addition, it should also display a splitting of the lines increasing with K, which offers a direct measure of the change in (A& between the degenerate levels. The observed features being weak and narrow, the rotational analysis has been limited to the J peaks only, even if in some cases a poorly resolved K structure is exhibited by the clusters examined. In spite of the discontinuity of the laser modes, a relevant number of P(J) and R(J) clusters of “hot” bands was measured. The identification as assisted by computed frequencies according to the parameters of Refs [3, 53 was checked, when possible, by the usual lower state combination differences, from the data concerning the v3 = 1 and vg = 1 states available in Ref. [9]. The rotational analysis was carried out using the polynomial method and the J peaks were fitted to the well known equation: vP.R=A+Bm+Cm2+Dm3,
(4)
with A=v,+[(A’-A”)-(B’-B”)]K’ B=B’+B” C=B’-B”-(D;-D;‘)?
Table 2. Line positions (cm- ‘) of the P(49, K) multiplet of the CF,“Br Y*+ v3 band
CF 3 *‘Br
-as
D= -2(D;+D;‘). The polynomial was expanded, successively introducing higher powers of m without improving the fit; the quartic term Em4 could not be determined significantly. If the additivity of tlAvB is assumed and the maximum of the J peaks is taken for K values not higher than 3, thequantity [(A’-A”)-(B’-B”)]K’ is very small (< -4 x 10m4 cm- ‘). Therefore, no correction for K structure of the frequencies appeared suitable. The molecular constants for the v2 +2v,-v, and “hot” bands of both isotopomers, V2+V3+V,--Vfj derived from the coefficients of the polynomial, are set out in Table 4 which also reports, for the sake of comparison, the B values of v3 and vg available
The vr + vj band of CF,Br
333
Table 4. Molecular constants (cm- ‘) for the vr + 2v, - vs and v2 + vj + vg- vg “hot” bands, and a”. Bvalues obtained in the v2+ vj region of CF,Br CFa8iBr
CF 379Br v,+2v,-v,* ; B” (D$+D;‘)x IO9
v,+vs+v~-vvs*
1120.9422 0.069776 (6) (9) 0.069944 (9) 0.0699346t 22 (4)
lo6 a;3 x lo6 afS3 x lo6 a$!s6x lo6 a! x lo6
v,+2v,-v,*
v,+v,+v,-v,*
1119.1293(4) 0.069129 (7) 0.069296 (7)
1116.5184(6) 0.069041(10) 0.069198(10) 0.0692116t 18(4)
1118.5593(6) 0.069710(10) 0.069867(10) 0.0698636t 25 (3)
198.0(S) 153.9(2) 204 (7) 292(10) 37(7)
203.3 (6) 159.3(3) 210(9) 276(10) 42 (9) 51.4t 119(10) 122.4.t
a$, x
a: x lo6
23 (3)
135(10) 121.8t
*Numbers in parentheses indicate the uncertainties (3 x SD). t From the microwave results of Ref. [9].
from microwave measurements [9]. For completeness, this table also includes the tlAsB coefficients derived from the bands analyzed in the present work. DISCUSSION
The complete analysis of the vs +v, region of CF,Br allowed us to identify more than 550 individual transitions for the main band and a considerable number of J peaks for the “hot” bands v1 + 2v, - vj and v2 + vj +v,-v, of both the isotopic modifications. The observed line positions, together with the assignments and the (O-C) values for all the bands investigated, are compiled in Tables 5-7, which are available as supplementary material.* In the refinement procedure of the v,+v, parameters (see Table 3), the ground state constants (B”, 0;’ and D;Ik) were fixed to the microwave values [9], and only the upper state constants and the band origin were allowed to change. The small quantity (Ok --Di) was significantly determined only for the CF,*lBr species and the obtained value was used, as a constrained constant, in the final least-squares fit of the CF,79Br data. The sextic coefficients were also tested without improving the results. Figure 3 shows a small portion of the observed and simulated spectrum near 1127 cm- ‘. As can be seen, the experimental features match quite well with the simulated ones even for the “hot” band characteristics, and this accounts for the good quality of the results achieved. As expected, the envelopes of the J clusters in the “hot” bands were found to be sensitive to the us *Tables 5-7 have been deposited with the British Library, Boston Spa, Wetherby, West Yorkshire, U.K., as supplementary publication No. 13009 (4 pp.).
-c8 value. While U* was well determined from the polynomial analysis, attempts to extract K“ by computing a series of J manifold contours were not successful. Since the J peaks are weak and narrow, not so precise values of a” could be obtained and therefore it was decided to fix this parameter to the corresponding value for the v2 + vJ band; such an approximation is legitimate if the additivity of aATE is assumed. In addition, in the simulation of v2 + vj + vi ’ - v,i ’ features, (A[)’ was supposed to be equal to (A[)” and the reproduction obtained was deemed sufficient for our present use. This procedure leads us to neglect the splitting of K transitions into + 1and - 1components, which would be small when the same degenerate vibration is involved in the lower and upper state and no perturbation occurs. As mentioned above, interactions of the v1 + vj level with the nearby states are also expected. The Fermi resonance with vi and 2~: is, however, global and it is accounted for by the effective J and K dependence of the rotational constants. Following the results on CF,Cl [lo], from the value of 2~: of CF,Br [l l] it is possible to estimate the band origin of 2vt2 (F 1095 cm-‘), if the anharmonicity contribution is assumed invariant in the CF,X series. On this basis, considering the reduced energy levels of v,+v, and 2v*’ 5 Ta AI=Ak= +2 type interaction is expected to occur causing a crossing around K = 23 in 2~: 2. At this stage, no appreciable rotational resonance effects have been detected for transitions with K =21 in the v2 + vs band. In conclusion the present investigation allowed us to obtain a well conditioned set of spectroscopic parameters for the v2 +v, combination and related “hot” bands. The band origin and the rotational constants Q” and us are in satisfactory agreement with
A. BALDACCI et al.
334
GF
a
R171)
RI701
CF_"Br
:
I(
R1541
RI531
RI521
cm’
1127 0074
I li27 2530
I
Fig. 3. Detail of the R branch spectrum of the CF,Br Ye +v, band near 1127 cm-‘. (a) Experimental spectrum (T1200 K, P2 1 Torr). (b) Simulated spectrum including “hot” bands. K structure of R 54-52 of CF,79Br is labelled. E = vZ + 2v, - vj, 79Br (R 4745 from left). F = v2 + 2v, - vl, “Br (R 63-60). G = v1 + Ye 79Br (R 67-64). +v,--v,,
those available from Refs [4, 51, but are more accurately determined; a good additivity in the aB values (see Table 4) was also found. The quartic coefficients for the v2 + v3 band of both isotopomers were determined for the first time.
[4]
H. BURGER, K. BURCZYK, P. SCHULZ and A. RUOFF, Spectrochim. Acta 38A, 627 (1982).
[S]
K. BURCZYK, H. BURGER, A. RUOFF and M. MORILLON-CHAPEY, Spectrochim. Acta 37A, 615 (1981). A. BALDACCI, S. GIORGIANNI, R. VISINONI and S. GHERSETTI, J. molec. Spectrosc, in press. G. GUELACHVILY and K. NARAHARI RAO, Handbook of Infrared Standards. Academic Press, New York (1986). J. P. SATTLER, T. L. WORCHESKY and W. L. LAFFERTY, J. molec. Spectrosc. 88, 364 (1981). J. H. CARPENTER, J. D. MUSE and J. G. SMITH, J. them. Sot. Faraday Trans. II 78, 139 (1982). S. GIORGIANNI, A. BALDACCI, R. VISINONI and S. GHERSETTI, J. molec. Spectrosc. 130, 783 (1988). H. BURGER, K. BURCZYK, H. HOLLENSTEIN and M. QUACK, MoIec. Phys. 55, 255 (1985).
[6] [7]
REFERENCES [I]
H. JONES, F. KOHLER and H. D. RUDOLPH, .I. molec. Spectrosc.63, 205 (1976). [2] K. BURCZYK, H. BURGER, A. RtJoFFand P. PINSON, .I. molec. Spectrosc. 77, 109 (1979). [3] A. BALDACCI, A. PASSERINIand S. CHERSETTI, J. molec. Spectrosc. 91, 103 (1982).
[8] [9] [lo] [ll]