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The experimental vibrational frequencies of s-trans-1,3-butadiene, for which the assignments are well-established, are used to determine the scale factors for its quantum mechanical force field obtained at the MP2/6-31G*//MP2/6-31G* level of theory. The scale factors are then transferred to the MP2/6-31G*//MP2/6-31G* force fields of the s-cis and s-gauche rotamers and their theoretical frequencies calculated. Comparison of the vibrational frequencies of these three species indicates a special region of the IR spectrum of 1,3-butadiene in the gas phase (720−790 cm-1) in which only a band attributable to the s-gauche rotamer should be present; i.e., it should be free both of the observed IR bands of the s-trans and of the calculated vibrational frequencies of the s-cis conformer. Investigation of the medium- and high-resolution IR spectra of 1,3-butadiene in the gas phase reveals the presence of a band at 749.22(20) cm-1 possessing the typical B contour (consistent with A symmetry, C 2 group). Rotational analysis of the medium-resolution spectrum of this band yields the rotational constants A‘‘ − B̄‘‘ = 0.4478(27) cm-1 and A‘ − B̄‘ = 0.4455(25) cm-1, only about one-third of the experimental values for s-trans-1,3-butadiene. This identifies the band as belonging to the high-energy conformer of 1,3-butadiene. The agreement between the experimental and theoretical values of the band center (749 vs 735 cm-1), the clear B type contour, and the extremely complicated character of the high-resolution spectrum of the band at 749.22 cm-1 strongly suggest that the geometry of the high-energy conformer of 1,3-butadiene in the gas phase is nonplanar s-gauche and not planar s-cis.

A systematic study of the 213.8-nm (zinc line) photochemistry of 1,3-butadiene has been made either in the absence or in the presence of various additives-such as radical scavengers (02, NO, DI) and collisional quenchers-in the gas phase (pressure between 1 and 500 Torr). The major fate of the photoexcited 1,3-butadiene molecule is isomerization to the 1,2-butadiene structure which may then decompose to methyl and C3H3 radicals ( = 0.64 ± 0.04 at 1 Torr of 1,3-butadiene). Minor processes include decomposition to the acetylene + ethylene couple ( = 0.22 ± 0.02) or to vinylacetylene ( = 0.038 ± 0.003) and molecular hydrogen. These two minor processes occur from different excited states. Some 2-butyne ( < 0.015) is formed by a unimolecular isomerization process. The photolysis of 1,3-butadiene-l,l,4,4-d¡, indicates that at least three different intermediates are involved in the formation of molecular ethylene and acetylene. The C3H3 radicals are not easily intercepted by DI: k(C3H3 + 1,3-butadiene)/A:(C3H3 + DI) = 0.09 ± 0.03. Also at 21 °C and for [DI]/ [ 1,3-butadiene] = 10, the highest ratio used, (3 1 ß + propyne)/^(CH3D) = 0.72 and a fraction of the C3H3 radicals are still not accounted for (reaction with 1,3-butadiene and/or recombination?). The relative energies obtained by ab initio RHF-SCF geometry optimizations for the doublet electronic state of the C3H3 radical structures are £(propargyl) < £(propyn-l-yI) < £(cyclopropen-l-yl) < £(allenyl). General valence bond geometry optimizations and a multiconfigurational self-consistent-field surface scan also show that the propargyl species (1 2B, state) is the lowest energy one. There are probably at least two distinct C3H3 radical structures (different states) present in the far-UV photolysis of 1,3-butadiene.

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Thc gas-phase photochlorination of CHC13 has been investigated between 303.2 and 425.5"K. The reaction was followed both by a conventional and a mass spectrometric technique. Rate measurements in steady light lead to where k2, k3, k7 and k8 refer to Rate nieasurernents in intermittent light give logiok3 = -5,000/4.576T+ 8.74,which combined with eqn. (i) and (ii) and the known value of k2 yields log10k7 = 10.8 and loglok8 -9.66, independent of temperature. Using the values of k2 and k3 and thermodynamic data, onc calculates log1& = -11,3Oo/4576T+8.65 and loglok5 = --2O,000/4~576T+10~93, for thc reverse of reactions (2) and (3) respectively. All rate constants are given in mole-' 1. sec-'.The rate constant for the gas-phase recombination of trichloromethyl radicals has been measured by Tedder and Walton who found log,,k8 * = 10.9 between 350 and 446°K. This value is in line with those observed for methyl and trifhoromethyl radicals 2* but is higher than would be expected from a comparison of the values observed for C2Cl,(8-6),4 C2HC1,(9*3) and C2H5(10*5) radicals. It is also higher than a proposed value for CC13 radicals in the gas-phase (8.8) '9 and observed values in the liquid phase (7-7+8.1).* It therefore seemed important to measure li, in a chemical system different from that used by Tedder and Walton (C,H,+ CC13Br + hv). The gas-phase photochlorination of chloroform was chosen for this purpose. This reaction has been investigated using steady illumination by Chiltz, Mahieu and marten^.^ Between 360 and 430"K, the initial rate of chlorination is given by V, = -d[ClJdt = I$k3[ClJ/(k8 + k7k3[C12]/k2[CHC13]);t 6) * All rate constants are given in mole* The values S598 (CCl,) = 70.8 cal/mole deg. and AH;, 298 (CC1,) = 18 kcal/mole were chosen as mean values of existing data.

Total geometry optimization and computation of the ab initio force fields for cyclopropene and six of its fluoro derivatives are carried out at the HF/6-311G* level. The HF/6-311G*//HF/6-311G* force field of cyclopropene is scaled using the empirical scale factors determined only from the experimental vibrational frequencies of the light isotopic species of cyclopropene. The scaled force field obtained is used to calculate the vibrational frequencies for seven deutero analogues of cyclopropene and six of its fluoro derivatives. The scale factors for the >CF2 moiety of 3,3-difluorocyclopropene are refined using the experimental vibrational frequencies of its light isotopic species. The refined set of scale factors is transferred to the HF/6-311G*//HF/6-311G* force field of 1,2,3,3-tetrafluorocyclopropene. The experimental vibrational frequencies of this molecule are used to refine the scale factors for the C−F moieties. The scale factors obtained, together with the scale factors for cyclopropene, are used to predict the vibrational frequencies of the 3-fluoro-, 1,3,3-trifluoro-, 1-fluoro-, and 1,2-difluorocyclopropenes. The vibrational problems for all the molecules just mentioned and for some of their deutero analogues are solved using the HF/6-311G*//HF/6-311G* force fields corrected by the corresponding refined sets of scale factors. The complete assignments of all the fundamental frequencies are given. Some peculiarities of the vibrational spectra of this molecular series are discussed.

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