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Free-jet high-resolution FTIR spectroscopy of the complex structure of the ν1 band of CF3I near 9 μm
Volume
156.number 6
CHEMICAL
2 I April 1989
PHYSICS LETTERS
FREE-JET HIGH-RESOLUTION FTIR SPECTROSCOPY OF THE COMPLEX STRUCTURE OF THE Y, BAND OF CF31 NEAR 9 pm Hans BORGER,
Annette
RAHNER,
Anorganische Chemie, FB 9. Universitiii, D-5600 Wuppertal I, Federal Republic
ofGermany
Andreas AMREIN, Hans HOLLENSTEIN and Martin QUACK Laboratoriumfir Physikalische Chemie? ETH Ziirich (Zentrum). CH-8092 Zurich, Switzerland Received 5 January
1989; in final form 4 February
1989
The vI band of trifluoromethyl iodide (CF,I) has been investigated by high-resolution (0.005 cm ’ ) FTIR spectroscopy of a supersonic free jet at a rotational temperature of 45 k 15 K. Narrow J-resolved Q( P, R) (J) clusters were found for 2~7 (a, = 1080.1502( 9) cm-‘), with considerable intensity due to anharmonic interaction with I,, The Q branch of v, (CO= 1075.658(4) cm-‘) is split by anharmonic resonance with vj+3yc3 (& = 1073.758( 15) cm-l, lV,,,,,=O.5367(8) cm-‘), a level crossing occurring near K= 8. The K& I2 levels of u,, associated with QQ branches at lower wavenumbers, appear to be, in addition, rotationally perturbed. Several of the previously observed coincidences with CO, laser lines are perfectly matched by the proposed model, and apparent inconsistencies explained. The implications for IR multiphoton excitation are discussed.
1. Introduction CFJ is one of the central model systems in IR multiphoton excitation and laser chemistry [ 1 1. Because of the ease in pumping the strong v, fundamental near 1075 cm-’ and the large isotope shift of this band [2], CFJ has been used for “C/“C isotope separation [3], production of laser emissions [ 41, quantitative studies of rates and their nonlinear intensity dependence [ 5-71, time-resolved observations [6-S], very low pressure photolysis investigations [9], cw laser pumping in supersonic jets [ lo], the study of IR spectra of highly excited CF31 after laser excitation 17,111, and of product translational energy distributions [ 12,13 1. CFJ has also been a model for theoretical investigations [ 141 and it has been the first case where semiquantitative predictions of the nonlinear case C/B transition were verified experimentally [ 6,14 1. Further progress towards a quantitative understanding of IR multiphoton excitation will be possible only with a better understanding of the spectroscopic states of this molecule [ 21. In previous investigation9 of the v, range of 12CFJ 0 009-2614/89/$ ( North-Holland
03.50 0 Elsevier Science Publishers Physics Publishing Division )
we have established the global Fermi resonance with 2~‘: and v2+ us and the striking differences compared to the spectrum of ’ 'CF,I [ 2,15 ] (extensive references to older studies with dispersive instruments can be found there). The central Q branch region of “Y,” was shown to be very complex and it appeared as if the QQ branches of u, consisted of two bunches slightly above and below 1075 cm-‘. The very close level v3 + 3 v, was suspected as one of the most likely candidates for the unexplained additional perturbations of v,. Furthermore, double resonance experiments, which gave very accurate wavenumbers for a few transitions belonging to different J’ and K’ values of vi and several of its hot bands, could not be brought into agreement with any reasonable rovibrational model [ 16-201 (the last of these provides a detailed discussion). In view of these important, open questions we have studied the spectrum of CFJ by supersonic jet FTIR spectroscopy with cooling to very low rotational temperatures [ 2 l23 1. We report here a preliminary analysis of these spectra, which provides the first clear proof of the resonance of u, with v) + 3 v; 3, which is essential for further understanding of the high-resolution IR B.V.
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spectrum CFjI.
and of the double-resonance
CHEMICAL
spectra
PHYSICS
of
2. Experimental Details of the supersonic jet FTIR facility in Zurich, including our BOMEM DA002 interferometric spectrometer, and of the experimental procedures were described previously [ 22,231. Around 1000 cm- ’ the system has a proven resolving power P/AC> 400 000. The wavenumber calibration was carried out with NzO and NH1 lines and should provide an absolute accuracy of better than 0.0005 cm- ’ [24]. The spectra analyzed below showed effective linewidths of 0.005 and 0.007 cm- ’ (fwhm ), which are in part due to Doppler broadening in the unskimmed jet. They were obtained by coadding between 50 and 60 scans (several hours of scanning time). We used a liquid-He-cooled Cu/Ge detector with an optical filter which allowed us to measure at the same time the whole Y,, u,+ u3 and uq ranges. The latter bands are well and easily understood and are reported in detail separately [ l&23,25 1. We used backing pressures of about 240 kPa and a circular nozzle (diameter 150 pm, thickness 13 pm). CF31 was prepared in Wuppertal from CF$OOAg and Iz and purified by trap to trap distillation. The jet facility included a recovery system, avoiding even small losses of the substance. Decomposition of CF31 in the pumping system was very minor, as evident from the colour of small amounts of I1 found upon recovery.
3. Results and discussion 3. I. Description of the spectrum and its interpretation
The 1082 to 1072 cm-’ portion of the experimental spectrum is displayed in fig. la, trace A. Above 1082 cm- ’ the spectrum continues with sharp, unresolved OR(J” ) clusters of the typical parallel band 2~:, which do not exhibit any noticeable K structure, while for fii 1072 cm-’ the signals approach more and more the noise level. The strong 558
LETTERS
21 April 1959
unresolved peak at 108 1 cm- ’ is assigned to the Q branch of 2 vi [ 21. We note that this narrow Q branch reveals wings both to the higher and lower wavenumbers. Thus, it should comprise (J, K) components (K subband origins) both to higher and lower wavenumbers from &. Extrapolating A,-&= -4x 10e4 cm-’ from CF,Cl (-3.8~10-~ cm-’ [ 141) and CF,Br (-3.9~ 1O-4 cm-’ [ 121) and transferring B,-&~-l.9X10-4 cm-’ from 13CF,I [2], one would expect for the parallel band V, a structure similar to that in fig. la, trace D, with a Q branch degraded to smaller wavenumbers. No such structure is found. Instead, a structured absorption extending from about 1075.2 cm-’ to higher wavenumbers, displaying strong-weak-weak intensity alternation and growing separation of K peaks with rapid depletion for Kz 9, is observed, fig. lb. It is correlated with QR and QP lines, which can be identified for K=O, 1 (unresolved), 2, 3, 4, 5 and 6, the latter overlapping with K=3 lines differing by one J unit. The absorption maximum of this Q branch portion near 1075.3 cm-’ corresponds to one of the Q branch maxima, in fact the strongest one observed in the spectrum at and below ambient temperature [ 2 ] _ A second absorption between about 1074.3 and 1074.8 cm-’ corresponding to the broad absorption in the spectrum shown in ref. [ 2 ] is clearly evident in fig. la, trace A. It is unstructured with the exception of a strong and narrow peak at 1074.60 cm-‘. Certainly these features are also part of the Q branch of v,, although they cannot be assigned at present. From the sharpness of the QQK branches at the higher wavenumbers and the absence of any J-dependent shifts of attached QP and QR lines we conclude that these K levels are not significantly rotationally perturbed. Their spread and degradation should therefore arise from an anharmonic interaction with a perturber whose &, value is smaller than that of Y, and which possesses such a large effective rotational constant A that this can hardly be due to only an A,-& term. Such a requirement for & is met by the perturber vj + 3 ZJ;.l. Assuming additivity of (A[=), and adopting (A1=j6= 2.627 ( 13) X 10p2 [26] the K separation of successive Q branches of this pseudoperpendicular perturber is close to +0.158 cm-‘. It can interact with V, anharmonitally, and indeed such an interaction explains and
Volume 156,number 6
CHEMICAL
2 1 April I989
F’HYSlCS LETTERS
K:6
I
I
1
I
1076 5
I
I
I
I
I
1076.0 ‘In-’
I
I
I
I
I
,
1075 s
KS 12 (1075.3cm-‘) and v,/v3+3vd3, Fig. I. (a) Part ofthejet spectrumshowing the Q branchesof2vy (1081 cm-‘), v,/v,+3v,$‘, Kz6 ( 1074.0-1074.8 cm-‘). (A) Observed spectrum, Kassignmentsare indicated; (B) computed spectrum, truncation KG 12 (assuming T,,,s45 K): (C) computed spectrum, Knot truncated (assuming T,,=45 K); (D) computed spectrum, WW.~=O (assuming T,,,= 45 K). (b) Detail of traces (A) and (B). Assignments of QQ branches and some QR,(J” ) lines arc indicated.
quantitatively reproduces the shape of the Q branches P 1075.2 cm- I, fig. 1, traces B and C. K crossing of v, and vR+ 3 v;’ occurs close to K= 8. If there were no further perturbations one would expect a head formed by the V, OQ branches near 1074.60 cm-‘, similar to that displayed in fig. 1,
traces B and C, K values of 10 to 20 being close to this head. Apparently this part of the Q branch is smeared out. Although it cannot be ruled out that high K values are somewhat less populated than one would expect for T,,=45 K, we believe that the higher K levels of V, belonging to the lower wave559
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CHEMICAL
number Q branch fraction are rotationally perturbed, and we propose an explanation at the end. All our tits of the rather unperturbed 2~: and vq bands in the same spectrum are consistent with a rather well defined rotational temperature 45 k 15 K (see also ref. [ 23 1, recently another measurement of u., by diode-laser jet spectroscopy has become available [27] ). 3.2. Numerical Jit
Ground state parameters were held fixed (table 1) . Excited state centrifugal distortion coefficients D” were fixed to their ground state values D,. The vibrational level v, was coupled to 2~: and V~+ 3 vc3 (both of A, symmetry species) by the offdiagonal elements of the energy matrix W,,, and W 13666,respectively (independent of J and K). The experimental data did not yet allow a relinement of the difference A,-& for the following reasons. (i) The K structure of 2 vt is governed both by the interaction with v, and v3+3v;’ and ASS-Ao. Since no individual K components were resolved, we have fixed the latter quantity to zero. Its estimation by a band contour simulation would be only marginal. (ii) The quantity Al -A,-, is, at least for low K values (K,< 12), correlated to W13666 and As long as we do not understand the 64&-)3~~~. K> 12 levels of v ,, we prefer to fix A, -A0 to a rea-
ofthe data
The excited state parameters V, + 3 VT3 were fitted using
of 2vy,
v, and
E(v,I,J,K)=Y,+(A,,-B,)K’+B,,J(J+l) -D$J’(J+
I )2-Dt;KJ(.J+
l)K’-D$#
- 2 (A&)KI.
(1)
Table I Ground and excited state parameters “CFJ,
2 I April 1989
PHYSICS LETTERS
of CF,I
ground state
v. (cm-‘) A,-& (10-4cm-‘) B,.-B, (lo-4cm’) (,4c’).!(cm-‘)
1080.1502(9) 0.0 c’ 0.917(8)
WI,, (cm-’ ) W 13666(cm-’ 1 W lhhhd(cm-‘)
2.1 e’
1075.658(4) -3.5 c’ -1.915(20)
1073.758(15) 0.0 C’ -2.48(5) 0.0724(g)
0.5367(g) 0.94 e’ %F,I h
v. (cm-‘) AI--A0 (1W4cm-‘) B,--B, (lo-4CW’)
I-,
286.303(3) -0.020( IO) -0.0335(5)
-0.503
” From electron diffraction structure, ref. [28]. ‘) Ref. [26]. ‘) From harmonic force field, ref. [ 21. d1This work. e’ Constrained. ‘I Ref. [29]. g, Ref. [2]. “’ Note that the sign in ref. [2] was misprinted.
560
“5
8)
v6
539.830(7)
-0.1408
(AC’),. xjc (cm-‘)
1048.382(4) -5.0” -1.97(10)
h,
b.E)
261.5(15) 0.0 -0.88548(3) 0.02627( 13)
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CHEMICAL
21 April 1989
PHYSICS LETTERS
sonable estimate, - 3.5 x 1Oe4 cm- ‘. We have made sure that the overall interpretation and the quality of the data fit is insensitive to meaningful variations of A, -&. Similar arguments hold for A3666-AD. Since the interaction between v, and 2 vg is global, Wls5 cannot be determined from wavenumber data. Accordingly, W,,,=2.1 t- 0.2 cm-’ was estimated by comparing intensities (fig. la). It is evident from trace D ( W,,,,,=O) that the intensity of 2vt depends both on W,,, and W,3666. The previous estimate [2], W,5,=2.6 cm-‘, is in full agreement with the present result which is based on a more complete model. A total of 469 pieces of data were included in the numerical fit, 0=3.2x lop3 cm-‘, to yield the excited state parameters in table 1. Although the K= 0 and 1 lines were not resolved, the maxima of the peaks were alternatively used as hypothetical K=O and K= 1 lines. This procedure worsens the overall o value of the fit, it improves, however, the significance of the parameters. Relevant parameters of 13CF31and of vg, v5 and vg of lZCFJ as well as those of ref. [ 51 are also set out in table 1 for comparison. Complete tables of line positions for these and other data will be given elsewhere [ 25 1.
Kc 8 and QQ branches of v, for K> 8 remains unexplained, and we suggest that rotational perturbations may be responsible for its unstructured appearance. First, one has to consider the well-established perturbations within the +, vg and vh levels, of which overtones and combinations create the ensemble of perturbers affecting v,. A Ak= AI= 2 1 Coriolis X, y interaction between vj and v6 was revealed by the rotational spectra of the ZJ~=1 and v,= 1 states [ 261. The vs fundamental was shown to be anharmonitally perturbed by v3 + v6, and strong evidence in favour of an additional rotational perturbation, most likely of Ak= AI= & 1 type, with 2~: or 2v$’ was found in the AK= - 1 Q branches of vs [ 21. These perturbations, although inevitably present, are not considered here because we believe that direct rotational perturbations of v, are more important. In fig. 2 we have illustrated for KG 24 some levels neglecting anharmonicity, and marked by large
3.3. Indentijcation of the perturber and further
interactions The initial assumption that v,+ 3vc3 is the perturber of the low-K levels of v, is confirmed by the independently refined parameters B,,- B, and (N=),m which are close to those expected from the v3 and vs data listed in table 1. Although vo is not known precisely, one may compare the v. value of the perturber, 1073.8 cm-‘, with the quantity (cm-‘)
=286.3-
l.5+784.5+6(xe6
=1069.3+6(x,,
+g,,)
+gh6).
With the quoted uncertainty of u!j’, 1.5 cm-’ [2], the sum 6(xh6+gb6) is) calculated to be 4.5k4.5 cm-‘. We assume gee=0 (ref. [30]; 0.2(2) cm-‘) and obtain xe6= +(X75*0.75 cm-’ (ref. [30]; +0.7(S) cm-‘). The lower-wavenumber part of the Q branch of v, which should comprise oQ branches of v3 + 3 v;’ for
I
KS0
>
5
10
15
20
Fig. 2. Reduced zero-order (J=O) upper-state energies for ui and some of its possible perturbers. These were generally constructed from the vib-rotational parameters of their increments assuming additivity and neglecting anharmonicity. Effective and deperturbed levelsare shown for v,, 24 and v,+~v;~, experimentally determined energy levels are marked by bold full lines, extrapolated effective levels by bold dashed lines. Possible anharmonic perturbations are designated by squares, rotational interactions by large open circles. Full dots refer to laser measurements described in the text.
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PHYSICS LETTERS
circles the crossings close to which rotational interactions of the Ak= AI= ? 1 type would come to resonance. In particular the crossing of v,(K) with v7 + 3 uz ’ (K+ 1) near K= 8 is suspected to cause rotational interactions, which, as illustrated in fig. 2 by the moderate slope of yl + 3 VZ’ (Kf 1), may be extended. The analysis of higher K levels remains a task for the future [ 25 1. 3.4. The double-resonance measurements There are a total of six transitions which, on the basis of their rotational constants B” or quadrupole coupling constants (eQq) “, involve the ground state [20]. Four of these (labelled 1, 3,4, 5 in fig. 2) can be pumped by the 9R(l6), 9R( 14), 9R( 12) lines respectively of the ‘rC1602 laser, while two others (denoted 2 and 6) coincide with the 9P( 14) line of the ‘2C’80z laser. The laser resonance results are collected in table 2. Although we have not made use of any of the laser coincidences as input data, our model perfectly matches the transitions I, 3 and, most likely, also 4. These three transitions have been assigned by Kohler, Jones and Rudolph [ 171 to uI, u,. and u,., and their attached Bcfr are in full agreement with our model: K= 2 mostly V, in character, K= 5 carries almost 1: 1 V, and ZJ,+ 3~;~ character, while QQ,6( 18) is suspected to be rotationally perturbed and there-
Table 2 CO, laser coincidences No.
in “CFJ
Laser line
fore reveals different B,,r and (eQq),w values. We confirm that v,. and u,. are indeed part of the V, system. Because of the 2.5x lo-’ cm-’ difference we have some reservations for transition 2 as our fit includes many K=4 lines. With regard to transition 6 we feel at present unable to offer a model which would be consistent with this transition. Finally, we agree with previous [ 2,201 conclusions that transition 5 cannot belong to v,, although it might well be associated with one of the above mentioned perturbers.
4. Conclusions (i) Rotational cooling in FTIR supersonic jet spectroscopy has simplified the spectrum in the very complex Q branch region of the “CF? symmetric stretching fundamental v,” to a sufficient degree to allow for a first rotational assignment and analysis. The unambiguous identification of a split Q branch for V, with a K structure shifted to the opposite direction from that expected or predicted from czAand c? is one of the fundamental achievements of the present study. Such split Q branches frequently prevent the understanding of fundamental transitions and a rather similar case is the CH stretching fundamental v1 in CHF3 [22,31,32]. (ii ) We have provided the first proof that an an-
and some derived parameters Wavenumber (cm-’ )
Pumped transition
1 2 3 4 5
9R( 16) 9P(l4) - I80 9R( 14) 9R(l4) . 9R(12)
1075.988 1073.579 1074.646 1074.646 ~073.278
O&(7) QP,( 16) “P,(6)
6
9P( 14) - IX0
1073.579
QP,*(l81
03 Uh v, f 3 uh, calculated il’ Ref. [20]. h’ Ref. [ 171. c’ Ref. [ 181. ‘I Calculated for v,+3vZ’. ‘I Ref. [26].
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21 April 1989
(eQq):a-(eQq)o F-B, (MHz)
QQ~e(15) “Q,(S)
-
4.85( 1 I) a) 5.08( 13) ‘) (6.2) a’ 23.72( 17) a’ 12.2(23) ”
(cm-‘) this work
- 1.93 s)
1075.988 1073.554 1074.642 1074.637 1075.247, 1072.877 ‘) 1073.896
-2.25 b, -0.84 b’ - 1.87 ‘)
4.60( 35) ” -1.28(11)” 0.63( 12) ” 0.6(5)
~,,lC
( 10m4 cm-‘)
- 0.03 e’ -0.86 u, -2.61 (ohs.-2.48(5))
Volume 156, number 6
CHEMICAL PHYSICS LETTERS
harmonic resonance occurs between “lr,” and v,-+ 3 va 3 in addition to the previously established resonances with 24 and v,+ v3 [ 2]_ Thus the “v,
fundamental” really corresponds to complex anharmanic motion involving all the low-frequency modes (only v4 is missing, as it occurs at higher frequency 1. The situation is similar to 13CFJI, but there are differences in detail (resonance with 4 ug [ 2 ] ). (iii) Our data provide the basis for an analysis of several lines from double-resonance measurements, which is largely consistent with the one given by Fawzy and Schwendemann [ 201, but we provide additional clues. (iv) The resonance of V, with v,+~Y:~ is likely to be of importance for IR multiphoton excitation because of the different (negative and positive) anharmonicities. Because of (Al’),, the subband origins of u3 + 3 vg show a large K dependence and thus the resonance will be retained also for higher vibrational excitation. The situation is sufficiently different from the one in 13CF31 to allow for an explanation of different behaviour under IR multiphoton excitation [ 11. (v) The rotational structure for K< 10 in v, is now quite well understood but more work is necessary to analyze the levels with K > 10 [ 25 1.
Acknowledgement Help from and discussions with U. Schmitt are gratefully acknowledged. Our work is supported financially by the Schweizerischer Nationalfonds and the Schweizerischer Schulrat, as well as by the Deutsche Forschungsgemeinschaft (SFB 42).
References [ 1 ] D.W. Lupo and M. Quack, Chem. Rev. 87 (1987) 181; M. Quack, Infrared Phys., in press. [2 ] H. Biirger, K. Burczyk, H. Hollenstein and M. Quack, Mol.
Phys. 55 (1985) 255. [3 I S. Bittenson and P.L. Houston, J. Chem. Phys. 67 (1977) 4819; M. Cauchetler, 0. Croix, M. Lute and S. Tistchenko, Ber. Bunsenges. Physik. Chem. 89 ( 1985 ) 290; M. Quack and R. Widmer. to be published. [4] J.J. Tieeand C. Wittig, J. Appl. Phys. 49 (1978)
61.
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[ 51 M. Quack and G. Seyfang, Chem. Phys. Letters 93 ( 1982) 442; J. Chem. Phys. 76 (1982) 955. [6] M. Quack, E. Sutcliffe, P. Hackett and D.M. Rayner, Faraday Discussions Chem. Sot. 82 ( 1986) 229. [ 7 I P.Dietrich, M. Quack and G. Seyfang, Faraday Discussions Chem. Sot. 82 ( 1986) 280; Infrared Phys., in press. [8] B. Abel. L. Brouwer, H. Herzog, H. Hippler and J. Tree, Chem. Phys. Letters 127 (1986) 541. [9] R.M. Robertson, D.M. Golden and M.J. Rossi, J. Chem. Phys., in press. IlO] C. Licdenbaum, S. Stoke and J. Reuss, Infrared Phys., in pIXSS.
Bagratashvili, S.I. Ionov, M.V. Kuzmin, VS. Letokhov, G.V. Mishakov and A.A. Stuchebrukhov. in: Recent advances in molecular reaction dynamics, eds, R. Vetter and J. Vigue (CNRS, Paris, 1986) p. 425; O.V. Boyarkin, S.I. lonov and V.N. Bagratashvili, Chem. Phys. Letters 146 (1988) 106; S.I. Ionov and V.N. Bagratashvili, Chem. Phyr. Letters 146 (1988) 596. [ 12lA.S. Sudbo, P.A. Schulz, E.R. Grant, Y.R. Shen and Y.T. Lee, J. Chem. Phys. 70 (1979) 912. [ 131 D.M. Rayner and P.A. Hackett, J. Chem. Phys. 79 (1983) 5414. [14]M.Quack,Chimia35 (1981)463. [ 151 H. Hollenstein, M. Quack, H. Biirger and K. Burczyk, to be published. [ 161 H. Jones and F. Kohler, J. Mol. Spectry. 58 ( 1975) 125. [ 171 F. Kohler, H. Jones and H.D. Rudolph, J. Mol. Spectry. 80 (1980) 56. [ 181 E. lbisch and U. Andresen, Z. Naturforsch. 37a ( 1982) 37 I [ 191 H.-M. Ritze and V. Stert, J. Mol. Spectry. 94 (1982) 215. [20] W. Fawzy and R.H. Schwendemann, J. Mol. Spectry. 120 (1986) 317. [2 1 ] A. Amrein, H. Hollenstein, P. Lecher, M. Quack, U. Schmitt and H. Biirger, Chem. Phys. Letters 139 ( 1987) 82. [22] A. Amrein, M. Quack and U. Schmitt, J. Phys. Chem. 92 ( 1988) 5455; to be published [23] A. Amrein, H. Hollenstein, M. Quack and U. Schmitt, Infrared Phys., in press. [24]G. Guelachvili and K.N. Rao, Handbook of infrared standards (Academic Press, New York, 1986). [ 251 A. Amrein, H. Hollenstein, M. Quack, H. Biirger and A. Rahner, work in progress. [26]S.W. Walters and D.H. Whiffen, J. Chem. Sot. Faraday Trans.1179(1983)841. [27] P.B. Davies, N.A. Martin and M.D. Nunes, Spectrochim. Acta, to be published. [ 281 V. Typke, M. Dakkouri and H. Oberhammer. J. Mol. Struct. 44 (1978) 85. [29] K. Burcryk and H. Biirger, Spectrochim. Acta 10A (1984) 929. [ 301 M.O. Bulanin, L.A. Zhigula, T.D. Kolomiitsova and D.N. Shchepkin, Opt. Spectry. 56 ( 1984) 405. [ 311 H.R. Dilbal and M. Quack, Chem. Phys. Letters 80 ( I98 I ) 439. [32]A.S. Pine, G.T. Fraser and J.M. Pliva, J. Chem. Phys. 89 (1988) 2720. [ 11 I V.N.
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PHYSICS LETTERS
FREE-JET HIGH-RESOLUTION FTIR SPECTROSCOPY OF THE COMPLEX STRUCTURE OF THE Y, BAND OF CF31 NEAR 9 pm Hans BORGER,
Annette
RAHNER,
Anorganische Chemie, FB 9. Universitiii, D-5600 Wuppertal I, Federal Republic
ofGermany
Andreas AMREIN, Hans HOLLENSTEIN and Martin QUACK Laboratoriumfir Physikalische Chemie? ETH Ziirich (Zentrum). CH-8092 Zurich, Switzerland Received 5 January
1989; in final form 4 February
1989
The vI band of trifluoromethyl iodide (CF,I) has been investigated by high-resolution (0.005 cm ’ ) FTIR spectroscopy of a supersonic free jet at a rotational temperature of 45 k 15 K. Narrow J-resolved Q( P, R) (J) clusters were found for 2~7 (a, = 1080.1502( 9) cm-‘), with considerable intensity due to anharmonic interaction with I,, The Q branch of v, (CO= 1075.658(4) cm-‘) is split by anharmonic resonance with vj+3yc3 (& = 1073.758( 15) cm-l, lV,,,,,=O.5367(8) cm-‘), a level crossing occurring near K= 8. The K& I2 levels of u,, associated with QQ branches at lower wavenumbers, appear to be, in addition, rotationally perturbed. Several of the previously observed coincidences with CO, laser lines are perfectly matched by the proposed model, and apparent inconsistencies explained. The implications for IR multiphoton excitation are discussed.
1. Introduction CFJ is one of the central model systems in IR multiphoton excitation and laser chemistry [ 1 1. Because of the ease in pumping the strong v, fundamental near 1075 cm-’ and the large isotope shift of this band [2], CFJ has been used for “C/“C isotope separation [3], production of laser emissions [ 41, quantitative studies of rates and their nonlinear intensity dependence [ 5-71, time-resolved observations [6-S], very low pressure photolysis investigations [9], cw laser pumping in supersonic jets [ lo], the study of IR spectra of highly excited CF31 after laser excitation 17,111, and of product translational energy distributions [ 12,13 1. CFJ has also been a model for theoretical investigations [ 141 and it has been the first case where semiquantitative predictions of the nonlinear case C/B transition were verified experimentally [ 6,14 1. Further progress towards a quantitative understanding of IR multiphoton excitation will be possible only with a better understanding of the spectroscopic states of this molecule [ 21. In previous investigation9 of the v, range of 12CFJ 0 009-2614/89/$ ( North-Holland
03.50 0 Elsevier Science Publishers Physics Publishing Division )
we have established the global Fermi resonance with 2~‘: and v2+ us and the striking differences compared to the spectrum of ’ 'CF,I [ 2,15 ] (extensive references to older studies with dispersive instruments can be found there). The central Q branch region of “Y,” was shown to be very complex and it appeared as if the QQ branches of u, consisted of two bunches slightly above and below 1075 cm-‘. The very close level v3 + 3 v, was suspected as one of the most likely candidates for the unexplained additional perturbations of v,. Furthermore, double resonance experiments, which gave very accurate wavenumbers for a few transitions belonging to different J’ and K’ values of vi and several of its hot bands, could not be brought into agreement with any reasonable rovibrational model [ 16-201 (the last of these provides a detailed discussion). In view of these important, open questions we have studied the spectrum of CFJ by supersonic jet FTIR spectroscopy with cooling to very low rotational temperatures [ 2 l23 1. We report here a preliminary analysis of these spectra, which provides the first clear proof of the resonance of u, with v) + 3 v; 3, which is essential for further understanding of the high-resolution IR B.V.
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spectrum CFjI.
and of the double-resonance
CHEMICAL
spectra
PHYSICS
of
2. Experimental Details of the supersonic jet FTIR facility in Zurich, including our BOMEM DA002 interferometric spectrometer, and of the experimental procedures were described previously [ 22,231. Around 1000 cm- ’ the system has a proven resolving power P/AC> 400 000. The wavenumber calibration was carried out with NzO and NH1 lines and should provide an absolute accuracy of better than 0.0005 cm- ’ [24]. The spectra analyzed below showed effective linewidths of 0.005 and 0.007 cm- ’ (fwhm ), which are in part due to Doppler broadening in the unskimmed jet. They were obtained by coadding between 50 and 60 scans (several hours of scanning time). We used a liquid-He-cooled Cu/Ge detector with an optical filter which allowed us to measure at the same time the whole Y,, u,+ u3 and uq ranges. The latter bands are well and easily understood and are reported in detail separately [ l&23,25 1. We used backing pressures of about 240 kPa and a circular nozzle (diameter 150 pm, thickness 13 pm). CF31 was prepared in Wuppertal from CF$OOAg and Iz and purified by trap to trap distillation. The jet facility included a recovery system, avoiding even small losses of the substance. Decomposition of CF31 in the pumping system was very minor, as evident from the colour of small amounts of I1 found upon recovery.
3. Results and discussion 3. I. Description of the spectrum and its interpretation
The 1082 to 1072 cm-’ portion of the experimental spectrum is displayed in fig. la, trace A. Above 1082 cm- ’ the spectrum continues with sharp, unresolved OR(J” ) clusters of the typical parallel band 2~:, which do not exhibit any noticeable K structure, while for fii 1072 cm-’ the signals approach more and more the noise level. The strong 558
LETTERS
21 April 1959
unresolved peak at 108 1 cm- ’ is assigned to the Q branch of 2 vi [ 21. We note that this narrow Q branch reveals wings both to the higher and lower wavenumbers. Thus, it should comprise (J, K) components (K subband origins) both to higher and lower wavenumbers from &. Extrapolating A,-&= -4x 10e4 cm-’ from CF,Cl (-3.8~10-~ cm-’ [ 141) and CF,Br (-3.9~ 1O-4 cm-’ [ 121) and transferring B,-&~-l.9X10-4 cm-’ from 13CF,I [2], one would expect for the parallel band V, a structure similar to that in fig. la, trace D, with a Q branch degraded to smaller wavenumbers. No such structure is found. Instead, a structured absorption extending from about 1075.2 cm-’ to higher wavenumbers, displaying strong-weak-weak intensity alternation and growing separation of K peaks with rapid depletion for Kz 9, is observed, fig. lb. It is correlated with QR and QP lines, which can be identified for K=O, 1 (unresolved), 2, 3, 4, 5 and 6, the latter overlapping with K=3 lines differing by one J unit. The absorption maximum of this Q branch portion near 1075.3 cm-’ corresponds to one of the Q branch maxima, in fact the strongest one observed in the spectrum at and below ambient temperature [ 2 ] _ A second absorption between about 1074.3 and 1074.8 cm-’ corresponding to the broad absorption in the spectrum shown in ref. [ 2 ] is clearly evident in fig. la, trace A. It is unstructured with the exception of a strong and narrow peak at 1074.60 cm-‘. Certainly these features are also part of the Q branch of v,, although they cannot be assigned at present. From the sharpness of the QQK branches at the higher wavenumbers and the absence of any J-dependent shifts of attached QP and QR lines we conclude that these K levels are not significantly rotationally perturbed. Their spread and degradation should therefore arise from an anharmonic interaction with a perturber whose &, value is smaller than that of Y, and which possesses such a large effective rotational constant A that this can hardly be due to only an A,-& term. Such a requirement for & is met by the perturber vj + 3 ZJ;.l. Assuming additivity of (A[=), and adopting (A1=j6= 2.627 ( 13) X 10p2 [26] the K separation of successive Q branches of this pseudoperpendicular perturber is close to +0.158 cm-‘. It can interact with V, anharmonitally, and indeed such an interaction explains and
Volume 156,number 6
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2 1 April I989
F’HYSlCS LETTERS
K:6
I
I
1
I
1076 5
I
I
I
I
I
1076.0 ‘In-’
I
I
I
I
I
,
1075 s
KS 12 (1075.3cm-‘) and v,/v3+3vd3, Fig. I. (a) Part ofthejet spectrumshowing the Q branchesof2vy (1081 cm-‘), v,/v,+3v,$‘, Kz6 ( 1074.0-1074.8 cm-‘). (A) Observed spectrum, Kassignmentsare indicated; (B) computed spectrum, truncation KG 12 (assuming T,,,s45 K): (C) computed spectrum, Knot truncated (assuming T,,=45 K); (D) computed spectrum, WW.~=O (assuming T,,,= 45 K). (b) Detail of traces (A) and (B). Assignments of QQ branches and some QR,(J” ) lines arc indicated.
quantitatively reproduces the shape of the Q branches P 1075.2 cm- I, fig. 1, traces B and C. K crossing of v, and vR+ 3 v;’ occurs close to K= 8. If there were no further perturbations one would expect a head formed by the V, OQ branches near 1074.60 cm-‘, similar to that displayed in fig. 1,
traces B and C, K values of 10 to 20 being close to this head. Apparently this part of the Q branch is smeared out. Although it cannot be ruled out that high K values are somewhat less populated than one would expect for T,,=45 K, we believe that the higher K levels of V, belonging to the lower wave559
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number Q branch fraction are rotationally perturbed, and we propose an explanation at the end. All our tits of the rather unperturbed 2~: and vq bands in the same spectrum are consistent with a rather well defined rotational temperature 45 k 15 K (see also ref. [ 23 1, recently another measurement of u., by diode-laser jet spectroscopy has become available [27] ). 3.2. Numerical Jit
Ground state parameters were held fixed (table 1) . Excited state centrifugal distortion coefficients D” were fixed to their ground state values D,. The vibrational level v, was coupled to 2~: and V~+ 3 vc3 (both of A, symmetry species) by the offdiagonal elements of the energy matrix W,,, and W 13666,respectively (independent of J and K). The experimental data did not yet allow a relinement of the difference A,-& for the following reasons. (i) The K structure of 2 vt is governed both by the interaction with v, and v3+3v;’ and ASS-Ao. Since no individual K components were resolved, we have fixed the latter quantity to zero. Its estimation by a band contour simulation would be only marginal. (ii) The quantity Al -A,-, is, at least for low K values (K,< 12), correlated to W13666 and As long as we do not understand the 64&-)3~~~. K> 12 levels of v ,, we prefer to fix A, -A0 to a rea-
ofthe data
The excited state parameters V, + 3 VT3 were fitted using
of 2vy,
v, and
E(v,I,J,K)=Y,+(A,,-B,)K’+B,,J(J+l) -D$J’(J+
I )2-Dt;KJ(.J+
l)K’-D$#
- 2 (A&)KI.
(1)
Table I Ground and excited state parameters “CFJ,
2 I April 1989
PHYSICS LETTERS
of CF,I
ground state
v. (cm-‘) A,-& (10-4cm-‘) B,.-B, (lo-4cm’) (,4c’).!(cm-‘)
1080.1502(9) 0.0 c’ 0.917(8)
WI,, (cm-’ ) W 13666(cm-’ 1 W lhhhd(cm-‘)
2.1 e’
1075.658(4) -3.5 c’ -1.915(20)
1073.758(15) 0.0 C’ -2.48(5) 0.0724(g)
0.5367(g) 0.94 e’ %F,I h
v. (cm-‘) AI--A0 (1W4cm-‘) B,--B, (lo-4CW’)
I-,
286.303(3) -0.020( IO) -0.0335(5)
-0.503
” From electron diffraction structure, ref. [28]. ‘) Ref. [26]. ‘) From harmonic force field, ref. [ 21. d1This work. e’ Constrained. ‘I Ref. [29]. g, Ref. [2]. “’ Note that the sign in ref. [2] was misprinted.
560
“5
8)
v6
539.830(7)
-0.1408
(AC’),. xjc (cm-‘)
1048.382(4) -5.0” -1.97(10)
h,
b.E)
261.5(15) 0.0 -0.88548(3) 0.02627( 13)
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PHYSICS LETTERS
sonable estimate, - 3.5 x 1Oe4 cm- ‘. We have made sure that the overall interpretation and the quality of the data fit is insensitive to meaningful variations of A, -&. Similar arguments hold for A3666-AD. Since the interaction between v, and 2 vg is global, Wls5 cannot be determined from wavenumber data. Accordingly, W,,,=2.1 t- 0.2 cm-’ was estimated by comparing intensities (fig. la). It is evident from trace D ( W,,,,,=O) that the intensity of 2vt depends both on W,,, and W,3666. The previous estimate [2], W,5,=2.6 cm-‘, is in full agreement with the present result which is based on a more complete model. A total of 469 pieces of data were included in the numerical fit, 0=3.2x lop3 cm-‘, to yield the excited state parameters in table 1. Although the K= 0 and 1 lines were not resolved, the maxima of the peaks were alternatively used as hypothetical K=O and K= 1 lines. This procedure worsens the overall o value of the fit, it improves, however, the significance of the parameters. Relevant parameters of 13CF31and of vg, v5 and vg of lZCFJ as well as those of ref. [ 51 are also set out in table 1 for comparison. Complete tables of line positions for these and other data will be given elsewhere [ 25 1.
Kc 8 and QQ branches of v, for K> 8 remains unexplained, and we suggest that rotational perturbations may be responsible for its unstructured appearance. First, one has to consider the well-established perturbations within the +, vg and vh levels, of which overtones and combinations create the ensemble of perturbers affecting v,. A Ak= AI= 2 1 Coriolis X, y interaction between vj and v6 was revealed by the rotational spectra of the ZJ~=1 and v,= 1 states [ 261. The vs fundamental was shown to be anharmonitally perturbed by v3 + v6, and strong evidence in favour of an additional rotational perturbation, most likely of Ak= AI= & 1 type, with 2~: or 2v$’ was found in the AK= - 1 Q branches of vs [ 21. These perturbations, although inevitably present, are not considered here because we believe that direct rotational perturbations of v, are more important. In fig. 2 we have illustrated for KG 24 some levels neglecting anharmonicity, and marked by large
3.3. Indentijcation of the perturber and further
interactions The initial assumption that v,+ 3vc3 is the perturber of the low-K levels of v, is confirmed by the independently refined parameters B,,- B, and (N=),m which are close to those expected from the v3 and vs data listed in table 1. Although vo is not known precisely, one may compare the v. value of the perturber, 1073.8 cm-‘, with the quantity (cm-‘)
=286.3-
l.5+784.5+6(xe6
=1069.3+6(x,,
+g,,)
+gh6).
With the quoted uncertainty of u!j’, 1.5 cm-’ [2], the sum 6(xh6+gb6) is) calculated to be 4.5k4.5 cm-‘. We assume gee=0 (ref. [30]; 0.2(2) cm-‘) and obtain xe6= +(X75*0.75 cm-’ (ref. [30]; +0.7(S) cm-‘). The lower-wavenumber part of the Q branch of v, which should comprise oQ branches of v3 + 3 v;’ for
I
KS0
>
5
10
15
20
Fig. 2. Reduced zero-order (J=O) upper-state energies for ui and some of its possible perturbers. These were generally constructed from the vib-rotational parameters of their increments assuming additivity and neglecting anharmonicity. Effective and deperturbed levelsare shown for v,, 24 and v,+~v;~, experimentally determined energy levels are marked by bold full lines, extrapolated effective levels by bold dashed lines. Possible anharmonic perturbations are designated by squares, rotational interactions by large open circles. Full dots refer to laser measurements described in the text.
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PHYSICS LETTERS
circles the crossings close to which rotational interactions of the Ak= AI= ? 1 type would come to resonance. In particular the crossing of v,(K) with v7 + 3 uz ’ (K+ 1) near K= 8 is suspected to cause rotational interactions, which, as illustrated in fig. 2 by the moderate slope of yl + 3 VZ’ (Kf 1), may be extended. The analysis of higher K levels remains a task for the future [ 25 1. 3.4. The double-resonance measurements There are a total of six transitions which, on the basis of their rotational constants B” or quadrupole coupling constants (eQq) “, involve the ground state [20]. Four of these (labelled 1, 3,4, 5 in fig. 2) can be pumped by the 9R(l6), 9R( 14), 9R( 12) lines respectively of the ‘rC1602 laser, while two others (denoted 2 and 6) coincide with the 9P( 14) line of the ‘2C’80z laser. The laser resonance results are collected in table 2. Although we have not made use of any of the laser coincidences as input data, our model perfectly matches the transitions I, 3 and, most likely, also 4. These three transitions have been assigned by Kohler, Jones and Rudolph [ 171 to uI, u,. and u,., and their attached Bcfr are in full agreement with our model: K= 2 mostly V, in character, K= 5 carries almost 1: 1 V, and ZJ,+ 3~;~ character, while QQ,6( 18) is suspected to be rotationally perturbed and there-
Table 2 CO, laser coincidences No.
in “CFJ
Laser line
fore reveals different B,,r and (eQq),w values. We confirm that v,. and u,. are indeed part of the V, system. Because of the 2.5x lo-’ cm-’ difference we have some reservations for transition 2 as our fit includes many K=4 lines. With regard to transition 6 we feel at present unable to offer a model which would be consistent with this transition. Finally, we agree with previous [ 2,201 conclusions that transition 5 cannot belong to v,, although it might well be associated with one of the above mentioned perturbers.
4. Conclusions (i) Rotational cooling in FTIR supersonic jet spectroscopy has simplified the spectrum in the very complex Q branch region of the “CF? symmetric stretching fundamental v,” to a sufficient degree to allow for a first rotational assignment and analysis. The unambiguous identification of a split Q branch for V, with a K structure shifted to the opposite direction from that expected or predicted from czAand c? is one of the fundamental achievements of the present study. Such split Q branches frequently prevent the understanding of fundamental transitions and a rather similar case is the CH stretching fundamental v1 in CHF3 [22,31,32]. (ii ) We have provided the first proof that an an-
and some derived parameters Wavenumber (cm-’ )
Pumped transition
1 2 3 4 5
9R( 16) 9P(l4) - I80 9R( 14) 9R(l4) . 9R(12)
1075.988 1073.579 1074.646 1074.646 ~073.278
O&(7) QP,( 16) “P,(6)
6
9P( 14) - IX0
1073.579
QP,*(l81
03 Uh v, f 3 uh, calculated il’ Ref. [20]. h’ Ref. [ 171. c’ Ref. [ 181. ‘I Calculated for v,+3vZ’. ‘I Ref. [26].
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21 April 1989
(eQq):a-(eQq)o F-B, (MHz)
QQ~e(15) “Q,(S)
-
4.85( 1 I) a) 5.08( 13) ‘) (6.2) a’ 23.72( 17) a’ 12.2(23) ”
(cm-‘) this work
- 1.93 s)
1075.988 1073.554 1074.642 1074.637 1075.247, 1072.877 ‘) 1073.896
-2.25 b, -0.84 b’ - 1.87 ‘)
4.60( 35) ” -1.28(11)” 0.63( 12) ” 0.6(5)
~,,lC
( 10m4 cm-‘)
- 0.03 e’ -0.86 u, -2.61 (ohs.-2.48(5))
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CHEMICAL PHYSICS LETTERS
harmonic resonance occurs between “lr,” and v,-+ 3 va 3 in addition to the previously established resonances with 24 and v,+ v3 [ 2]_ Thus the “v,
fundamental” really corresponds to complex anharmanic motion involving all the low-frequency modes (only v4 is missing, as it occurs at higher frequency 1. The situation is similar to 13CFJI, but there are differences in detail (resonance with 4 ug [ 2 ] ). (iii) Our data provide the basis for an analysis of several lines from double-resonance measurements, which is largely consistent with the one given by Fawzy and Schwendemann [ 201, but we provide additional clues. (iv) The resonance of V, with v,+~Y:~ is likely to be of importance for IR multiphoton excitation because of the different (negative and positive) anharmonicities. Because of (Al’),, the subband origins of u3 + 3 vg show a large K dependence and thus the resonance will be retained also for higher vibrational excitation. The situation is sufficiently different from the one in 13CF31 to allow for an explanation of different behaviour under IR multiphoton excitation [ 11. (v) The rotational structure for K< 10 in v, is now quite well understood but more work is necessary to analyze the levels with K > 10 [ 25 1.
Acknowledgement Help from and discussions with U. Schmitt are gratefully acknowledged. Our work is supported financially by the Schweizerischer Nationalfonds and the Schweizerischer Schulrat, as well as by the Deutsche Forschungsgemeinschaft (SFB 42).
References [ 1 ] D.W. Lupo and M. Quack, Chem. Rev. 87 (1987) 181; M. Quack, Infrared Phys., in press. [2 ] H. Biirger, K. Burczyk, H. Hollenstein and M. Quack, Mol.
Phys. 55 (1985) 255. [3 I S. Bittenson and P.L. Houston, J. Chem. Phys. 67 (1977) 4819; M. Cauchetler, 0. Croix, M. Lute and S. Tistchenko, Ber. Bunsenges. Physik. Chem. 89 ( 1985 ) 290; M. Quack and R. Widmer. to be published. [4] J.J. Tieeand C. Wittig, J. Appl. Phys. 49 (1978)
61.
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[ 51 M. Quack and G. Seyfang, Chem. Phys. Letters 93 ( 1982) 442; J. Chem. Phys. 76 (1982) 955. [6] M. Quack, E. Sutcliffe, P. Hackett and D.M. Rayner, Faraday Discussions Chem. Sot. 82 ( 1986) 229. [ 7 I P.Dietrich, M. Quack and G. Seyfang, Faraday Discussions Chem. Sot. 82 ( 1986) 280; Infrared Phys., in press. [8] B. Abel. L. Brouwer, H. Herzog, H. Hippler and J. Tree, Chem. Phys. Letters 127 (1986) 541. [9] R.M. Robertson, D.M. Golden and M.J. Rossi, J. Chem. Phys., in press. IlO] C. Licdenbaum, S. Stoke and J. Reuss, Infrared Phys., in pIXSS.
Bagratashvili, S.I. Ionov, M.V. Kuzmin, VS. Letokhov, G.V. Mishakov and A.A. Stuchebrukhov. in: Recent advances in molecular reaction dynamics, eds, R. Vetter and J. Vigue (CNRS, Paris, 1986) p. 425; O.V. Boyarkin, S.I. lonov and V.N. Bagratashvili, Chem. Phys. Letters 146 (1988) 106; S.I. Ionov and V.N. Bagratashvili, Chem. Phyr. Letters 146 (1988) 596. [ 12lA.S. Sudbo, P.A. Schulz, E.R. Grant, Y.R. Shen and Y.T. Lee, J. Chem. Phys. 70 (1979) 912. [ 131 D.M. Rayner and P.A. Hackett, J. Chem. Phys. 79 (1983) 5414. [14]M.Quack,Chimia35 (1981)463. [ 151 H. Hollenstein, M. Quack, H. Biirger and K. Burczyk, to be published. [ 161 H. Jones and F. Kohler, J. Mol. Spectry. 58 ( 1975) 125. [ 171 F. Kohler, H. Jones and H.D. Rudolph, J. Mol. Spectry. 80 (1980) 56. [ 181 E. lbisch and U. Andresen, Z. Naturforsch. 37a ( 1982) 37 I [ 191 H.-M. Ritze and V. Stert, J. Mol. Spectry. 94 (1982) 215. [20] W. Fawzy and R.H. Schwendemann, J. Mol. Spectry. 120 (1986) 317. [2 1 ] A. Amrein, H. Hollenstein, P. Lecher, M. Quack, U. Schmitt and H. Biirger, Chem. Phys. Letters 139 ( 1987) 82. [22] A. Amrein, M. Quack and U. Schmitt, J. Phys. Chem. 92 ( 1988) 5455; to be published [23] A. Amrein, H. Hollenstein, M. Quack and U. Schmitt, Infrared Phys., in press. [24]G. Guelachvili and K.N. Rao, Handbook of infrared standards (Academic Press, New York, 1986). [ 251 A. Amrein, H. Hollenstein, M. Quack, H. Biirger and A. Rahner, work in progress. [26]S.W. Walters and D.H. Whiffen, J. Chem. Sot. Faraday Trans.1179(1983)841. [27] P.B. Davies, N.A. Martin and M.D. Nunes, Spectrochim. Acta, to be published. [ 281 V. Typke, M. Dakkouri and H. Oberhammer. J. Mol. Struct. 44 (1978) 85. [29] K. Burcryk and H. Biirger, Spectrochim. Acta 10A (1984) 929. [ 301 M.O. Bulanin, L.A. Zhigula, T.D. Kolomiitsova and D.N. Shchepkin, Opt. Spectry. 56 ( 1984) 405. [ 311 H.R. Dilbal and M. Quack, Chem. Phys. Letters 80 ( I98 I ) 439. [32]A.S. Pine, G.T. Fraser and J.M. Pliva, J. Chem. Phys. 89 (1988) 2720. [ 11 I V.N.
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