The infrared laser spectrum of CH2CFCl near 1190 cm−1; rovibrational study of the C–F stretching mode

Journal of Molecular Structure 599 (2001) 31±38

www.elsevier.com/locate/molstruc

The infrared laser spectrum of CH2yCFCl near 1190 cm 21; rovibrational study of the C±F stretching mode q A. Pietropolli Charmet, P. Stoppa, S. Giorgianni*, S. Ghersetti Dipartmento di Chimica Fisica, UniversitaÁ Ca' Forscari di Venezia, Dorsoduro 2137, I-30123 Venezia, Italy Received 26 June 2000; accepted 6 September 2000

Abstract The infrared spectrum of 1-chloro-1-¯uoroethylene has been investigated in the n5 band region between 1168 and 1209 cm 21, at a resolution of about 0.002 cm 21, employing a tunable diode laser spectrometer. This vibration of symmetry species A 0 ; corresponding to the C±F stretching mode, yields a strong a/b-hybrid band with contributions of comparable intensity from both the components. Most of the J and K structure was resolved in different subbranches, and the rovibrational analysis led to the identi®cation of 2900 lines (J # 71; Ka # 27) for the n5 band of 35Cl species, whose origin is at 1188.455 cm 21. The P- and R-branch spectrum with high J value is dominated by bandheads spaced by about 2C and consisting of a series of lines arising from oblate-type near-degeneracy. Using Watson's A-reduction Hamiltonian in the I r representation, a set of spectroscopic parameters up to the quartic centrifugal distortion terms was obtained for the n5 ˆ 1 state. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Diode laser infrared spectrum; 1-Chloro-1-¯uoroethylene; Rovibrational analysis; C±F stretching mode

1. Introduction High resolution infrared spectra of chloro¯uoromethane [1] and cis-1-chloro-2-¯uoroethylene [2,3] have been recently investigated in different regions of atmospheric interest and the results obtained led to the determination of accurate constants for several fundamentals and to the understanding of the interaction mechanisms for the perturbations observed. As a part of a more complete study of the infrared characteristics of chloro¯uoroethylenes, the analysis has now been extended to 1-chloro-1-¯uoroethylene. q This paper is dedicated to Professor Alfred Bauder in appreciation of his signi®cant contributions to the ®eld of microwave spectroscopy. * Corresponding author. Tel.: 139-41-2578608; fax: 139-412578594. E-mail address: [email protected] (S. Giorgianni).

Many of these compounds play a signi®cant role in environmental processes and high-resolution measurements are the best source of information for detecting trace gases. Low-resolution infrared and Raman spectra of CH2yCFCl were investigated a long time ago [4,5] and band centers have been determined for several absorptions including fundamentals, overtones and combination bands. Earlier studies of the microwave spectra limited to relatively low J transitions, were concerned with obtaining the ground state rotational constants and chlorine nuclear quadrupole coupling constants [6]. Recently, measurements extended to transitions with higher quantum numbers allowed to re®ne the ground state parameters up to the sextic distortion terms for both chlorine isotopic species [7]. Results concerning the two dimensional microwave

0022-2860/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0022-286 0(01)00835-3

32

A. Pietropolli Charmet et al. / Journal of Molecular Structure 599 (2001) 31±38

Fourier transform spectroscopy were also presented during recent years by Bauder et al. [8,9]. The present work, which represents the ®rst high-resolution infrared study of CH2yCFCl, deals with the analysis of the rovibrational details of the C±F stretching fundamental near 1190 cm 21. 2. Experimental details

Fig. 1. Molecular geometry and symmetry information for 1-chloro1-¯uoroethylene.

The gas sample of CH2yCFCl (purity 99%) was purchased from Peninsular Chemical Research, Inc., and was used without further puri®cation. The spectra were recorded in the range 1168±1209 cm 21, at a resolution of about 0.002 cm 21, employing the tunable diode laser spectrometer available in our laboratory. The instrument is interfaced to a PC that provides data acquisition, storage and conversion into transmittance of the spectra. The measurements were performed at a pressure of 0.2 mbar with a 49 cm cell cooled at 240 K in order to decrease hot band structures. Absorption lines were calibrated using a semiautomatic routine based on interference patterns from a 2.59 cm germanium eÂtalon (free spectral range ù 0:0475 cm 21) and on

Fig. 2. Section of the R branch spectrum of CH2yCFCl n5 band (T ˆ 240 K; P ù 0:2 mbar; 49 cm cell) near 1191 cm 21 showing the Ka structure of the QRK(8, 9) groups of 35Cl species; dotted marker refers to computed wavenumbers. Asymmetry splitting for Ka # 5 is observed.

A. Pietropolli Charmet et al. / Journal of Molecular Structure 599 (2001) 31±38

33

Fig. 3. Details of the P-branch spectrum of CH2yCFCl n5 band (T ˆ 240 K; P ù 0:2 mbar; 49 cm cell) near 1175 cm 21 showing the line sequences in the QPK(J ˆ 53±51) bandheads of 35Cl isotopomer. The 1 and 2 signs of Ka values correspond to …Ka 1 Kc ˆ J† or …Ka 1 Kc ˆ J 1 1†; respectively.

Fig. 4. R-branch section of CH2yCFCl n5 band (T ˆ 240 K; P ù 0:2 mbar; 49 cm cell) near 1206 cm 21 illustrating the J structure of the 35Cl R RK(J) series of the b-type component; dotted marker refers to computed wavenumbers. The bandheads degrading towards lower wavenumbers show a sequence of lines with J decreasing by 2 and Ka increasing by 1.

34

A. Pietropolli Charmet et al. / Journal of Molecular Structure 599 (2001) 31±38

Fig. 5. Diagram of assigned transitions for the CH2yCFCl n5 band as a function of J and Ka : The number for each couple …J; Ka † indicates the number of transitions reaching the same even upper level …Ka 1 Kc ˆ J†:

A. Pietropolli Charmet et al. / Journal of Molecular Structure 599 (2001) 31±38

the wavenumbers of N2O [10] and SO2 [11] standard lines. The absolute wavenumber accuracy of the measurements is estimated to be usually better than 0.0015 cm 21. 3. Results and discussion 1-Chloro-1-¯uoroethylene is a planar asymmetric rotor …k ˆ 20:54† belonging to the symmetry point group Cs : The molecular geometry together with the symmetry information is reported in Fig. 1. The molecular symmetry plane contains the a and b principal axes, while the axis c, corresponding to the largest moment of inertia, is perpendicular to this plane. The twelve fundamentals are classi®ed as nine …n1 ±n9 †; of symmetry species A 0 which give rise to a/b-hybrid bands and three of symmetry species A 00 …n10 ±n12 †; which produce c-type band contours. The n5 vibration, approximately representing the C±F stretching mode, yields a strong absorption at 1188 cm21. A low-resolution spectrum of this fundamental shows a typical a/b-hybrid band for which the two components are of comparable intensity; the band envelope exhibits a clear Q branch degrading towards lower wavenumbers and well de®ned P and R branches. An interesting observation is concerned with the high frequency side of the band, where weak features around 1220 cm 21, probably arising from the expected n7 1 n12 and/or 2n11 vibrations, are also present. The high-resolution ®ne structure indicates that, in the n5 hybrid band, the a-type lines overcrowd the central region of the absorption while the b-type features spread all over the spectral range. Following the usual selection rules for a- (DJ ˆ 0; ^1; DKa ˆ 0; DKc ˆ ^1) and b-type …DJ ˆ 0; ^1; DKa ˆ DKc ˆ ^1† transitions, several even and odd subbands are expected in the P, Q and R branches depending on (K 00a 1 K 00c ˆ J 00 ) or …K 00a 1 K 00c ˆ J 00 1 1†; respectively. 3.1. Interpretation of the spectrum and assignments The spectral analysis started with the identi®cation of the more prominent features belonging to the atype component. The Q branch is not easy to be analyzed since the structure, degrading to lower wavenumbers, is strongly overlapped in the eQ(0,1) and o Q(0, 2 1) subbranches. The band origin, estimated

35

from the high wavenumber edge of the Q branch, combined with the ground state parameters [7] allowed the identi®cation near the band center of several QPK(J) and QRK(J) groups, approximately separated by 2B ˆ …B 1 C† ˆ 0:28 cm21 : For low J values, the assignments of the resolved details within each group are suggested by a certain degree of regularity, being separated the consecutive unsplit features by about ‰…A 0 2 A 00 † 2 …B 0 2 B 00 †Š…2K 1 1†: As an example, a section of the R branch near 1191 cm 21 showing the ®ne structure of the QRK(8, 9) manifolds is illustrated in Fig. 2. The effect of the asymmetry splitting, evident for Ka # 5; gives rise to an irregular numbering of the lines with the even component located at the side of higher wavenumbers. Not surprisingly, as one moves away from the band center to increasing J values, the lines belonging to neighboring manifolds overlap each other producing a complex structure which is dif®cult to be identi®ed. Furthermore, due to a larger asymmetry splitting, the transitions with low Ka values produce lines spreading without any regular pattern. Valuable information for the analysis of the spectrum is provided mostly from the ®ne structure of groups with high J values and low Ka values, characteristic of the spectra of planar molecules. The spectrum is dominated in the P and R branches by distinct bandheads separated by about 2C ˆ 0:23 cm21 and consists of a series of transitions characterized by DJ ˆ 21; DKa ˆ 1 and DKc ˆ 22 between successive lines each involving the almost degenerate levels with Ka1 and …Ka 1 1†2 : Features of this type were investigated by Borchert [12] and Kisiel [13] and are due to the near coincidence of transitions between energy levels which become degenerate in the oblate symmetric top limit. A specimen spectrum is reproduced in Fig. 3, where a section of the P branch near 1175 cm 21 exhibits the resolved J lines in the Q PK …J ˆ 53±51† bandheads. The transition which starts the series has J 00 ˆ K 00c and the degradation towards the higher frequency side proceeds with the line sequences of the two degenerate even and odd components given by …J 2 k 2 1†k;J22k21 à …J 2 k†k;J22k ; …J 2 k 2 1†k11;J22k21 à …J 2 k†k11;J22k ; where k ˆ 0; 1; 2; 3; ¼

36

A. Pietropolli Charmet et al. / Journal of Molecular Structure 599 (2001) 31±38

Fig. 6. Diagram of assigned transitions for the CH2yCFCl n5 band as a function of J and Ka : The number for each couple …J; Ka † indicates the number of transitions reaching the same odd upper level …Ka 1 Kc ˆ J 1 1†:

A. Pietropolli Charmet et al. / Journal of Molecular Structure 599 (2001) 31±38

The identi®cation of the b-type transitions took place after predicted values were obtained using the constants from the a-type components. The PP and RR subbranches give rise to weak lines and for high J values, produce distinct groups with peculiar patterns turning out to be helpful in assigning lines with high Ka values. A section of the b-type RR region near 1206 cm 21 is reported in Fig. 4, which shows different series of RRK(J) features. In each subgroup, the sequence of the lines occurs with J values decreasing by 2 and Ka values increasing by 1; thus the degradation towards the lower frequency side proceeds with transitions having the ground state numbers …J; Ka †; …J 2 2; Ka 1 1†; …J 2 4; Ka 1 2†; …J 2 6; Ka 1 3†;¼ As a ®nal point, it is worthwhile to note that the weak transitions of the 37Cl species could not be clearly recognized among the much stronger lines of the main isotopomer, and therefore, no rotational analysis has been attempted for the less abundant isotopomer. 3.2. Data analysis The rovibrational analysis was performed adopting a fairly common procedure. The ground and excited rotational energy levels were computed using the Watson's A-reduction Hamiltonian up to the sixth order in the I r representation Hˆ

1 2

…B 1 C†P2 1 ‰A 2

1 2

…B 1 C†ŠP2a 2 DJ P4

2 DJK P2 P2a 2 DK P4a 1 F J P6 1 F JK P4 P2a 1 F KJ P2 P4a 1 F K P6a 1 ‰ 12 …B 2 C† 2 2dJ P2 1 2fJ P4 Š…P2b 2 P2c †   i1 h ; 1 2dK P2a 1 fJK P2 P2a 1 fK P4a ; P2b 2 P2c where P is the operator of the total angular momentum and Pa ; Pb and Pc are its components along the principal inertial axes in the molecular-®xed coordinate system. As mentioned earlier, considerable help in the assignment of the rotational details was provided from the identi®cation of the ®ne structure in manifolds with medium J value and low Ka quantum numbers, characteristic of spectra of planar

37

molecules; these features arise from overlaps of transitions between energy level pairs that become degenerate at the oblate symmetric top limit. Preliminary line assignments were ®tted by a least squares routine, keeping ®xed the ground state parameters of Ref. [7] and re®ning the upper state constants together with the band origin. The obtained values allowed a better estimate of transitions with different J and Ka values and the re®ning procedure was iteratively applied until the analysis was completed. The identi®cation of the transitions did not prove to be very straightforward because the spectrum is quite complex due to weak features coming from `hot bands', combination bands, and 37 Cl isotopomer. The whole analysis led to the assignment of 2574 atype …J # 71; Ka # 26† and 326 b-type …J # 40; Ka # 27† transitions mostly occurring in the P and R branches. Few details of the a-type component have been considered in the very dense Q branch; most of the lines are composed of a large number of unresolved transitions. RQK(J) and PQK(J) b-type features were dif®cult to locate since they tend to get lost in the background of the weak lines. For the sake of completeness, the assigned transitions given as a function of J 0 and K 0a are included in Figs. 5 and 6. Each value in the diagrams refers to the number of the observed transitions corresponding to a given upper state level. The lines included in the ®t comprise the whole set of even and odd transitions. Since the asymmetry parameter …k ˆ 20:54† is too far from that of a prolate symmetric top, the asymmetric splitting associated with each Ka $ 1 level is expected to occur for low J values; for example, for Ka ˆ 6; the asymmetry splitting has been found for J $ 10: Unit weight was given to lines appearing as single transitions, while blended or scarcely resolved features were weighted with a factor of 0.1; badly overlapped characteristics were not considered and hence eliminated from the ®t. The parameters obtained, along with those adopted for the ground state [7], are summarized in Table 1; the standard deviation from the ®nal ®t of 2900 transitions is about 8 £ 1024 cm21 : Attempts to re®ne the sextic terms were not successful and therefore in the ®nal ®t, these coef®cients were constrained to their ground state values. The excited rotational and centrifugal distortion terms, listed in Table 1, compare reasonably well with those of the ground state with

38

A. Pietropolli Charmet et al. / Journal of Molecular Structure 599 (2001) 31±38

Table 1 Molecular constants (cm 21) for the n5 band of CH2yCF 35Cl (quoted uncertainties are one standard deviation in units of the last signi®cant digit) Ground state a

n5 ˆ 1

n0 1188.45497(5) A 0.35630155 0.35531023(43) B 0.170189565 0.169775180(176) C 0.115023223 0.114632354(62) DJ £ 107 0.47209 0.47960(55) DJK £ 106 0.16647 0.19852(35) DK £ 106 0.16775 0.13070(77) dJ £ 107 0.16198 0.16559(29) dK £ 106 0.18109 0.16663(27) No. of data 2900 J 00 max 71 K 00a max 27 s £ 10 3 0.807 F J £ 1013 ˆ 0:36; F JK £ 1012 ˆ 0:44; F KJ £ 1012 ˆ 20:37; F K £ 1012 ˆ 0:80; fJ £ 1013 ˆ 0:12; fJK £ 1012 ˆ 0:33; fK £ 1011 ˆ 0:31 b a

From Ref. [7]. Ground state sextic coef®cients (Ref. [7]) ®xed for the upper levels. b

the exception of DJK and DK ; where signi®cant differences have been observed. This behavior is probably due to the existence of weak Coriolis and/or Fermi interactions, which may be expected to be of importance in the region investigated considering the symmetry of the close-lying vibrational levels 2n11 …A 0 † and n7 1 n12 …A 00 †: 4. Conclusions This work reports the ®rst high-resolution infrared work of 1-chloro-1-¯uoroethylene concerning the study of the C±F stretching mode. Many lines with

high J and Ka values have been identi®ed in the a- and b-type components of the n5 band of the CH2yCF 35Cl isotopomer. A larger change between the ground and upper state values of DJK and DK parameters suggests that the n5 ˆ 1 state could be slightly perturbed by close lying vibrational levels.

Acknowledgements The authors thank M. Pedrali for assistance in taking the spectra. Financial support by MURST, Rome, is gratefully acknowledged.

References [1] A. Baldacci, P. Stoppa, S. Giorgianni, S. Ghersetti, J. Mol. Spectrosc. 194 (1999) 73. [2] P. Stoppa, S. Giorgianni, R. Visinoni, S. Ghersetti, Mol. Phys. 97 (1999) 329. [3] P. Stoppa, A. Pietropolli Charmet, S. Giorgianni, S. Ghersetti, Phys. Chem. Chem. Phys. 2 (2000) 1649. [4] J.R. Nielsen, J.C. Albright, J. Chem. Phys. 26 (1957) 1566. [5] D.E. Mann, N. Acquista, E.K. Plyler, J. Chem. Phys. 23 (1955) 2122. [6] R.G. Stone, W.H. Flygare, J. Mol. Spectrosc. 32 (1969) 233. [7] J.L. Alonso, A.G. Lesarri, L.A. Leal, J.C. LoÁpez, J. Mol. Spectrosc. 162 (1993) 4. [8] B. Vogelsanger, M. Andrist, A. Bauder, Chem. Phys. Lett. 144 (1988) 180. [9] B. Vogelsanger, A. Bauder, J. Chem. Phys. 92 (1990) 4101. [10] A.G. Maki, J.S. Wells, Wavenumber calibration tables from heterodyne frequency measurements, Special Publication 821, National Institute of Standards and Technology, Washington, DC, 1991. [11] HITRAN 1996 molecular spectroscopic database, available from the Atomic and Molecular Physics Division, HarvardSmithsonian Center for Astrophysics. [12] S.J. Borchert, J. Mol. Spectrosc. 57 (1975) 312. [13] Z. Kisiel, J. Mol. Spectrosc. 144 (1990) 381.