Vibrational spectra and assignments for 1-fluoro- and 1-chlorocyclobutenes: revised assignment for cyclobutene

Abstract

3549Mass spectroscopic Knudsen cell studies of the vapor over P4OI0(H) at 150, 315, and 400 OC have shown that P4OIo undergoes a phase transition which lowers the vapor pressure at 300 OC and that the vapor is composed of P4O10, P409, and P408 molecules with an approximate ratio of 10:3:1; furthermore, the decomposition products P408 and P407 showed increased concentration with higher Knudsen cell temperat~res.'~ While species such as P30e7' were produced by 40-eV electron impact, they showed no temperature dependence and were unique to mass spectroscopic studies. The P202-s+ ions were also produced by the electron impact process, as indicated by their high appearance potentials, but they showed increased intensities a t higher temperatures, corroborating the present matrix infrared observation that P40s and P407 are more prone to unimolecular cage decomposition than P4Olo. ConclusionsThe YAG laser evaporation of hexagonal (molecular) P4Olo produced mainly P4OIo with small yields of the P20s and POz decomposition products. The depolymerization of orthorhombic (covalent network) P4O10 with laser ablation gave a randomized distribution of PO, PO2, P20, P204, and P20s fragmentation (19) (a) Muenow, D. W.; Uy, 0. M.; Margrave, J. L. J. Inorg. Nucl. Chem. 1970, 32, 3459. (b) Hashizume, A.; Wasada, N.; Tsuchiya, T. Bull. Chem. SOC. Jpn. 1966,39, 150.products, where the smaller fragments had high intensities, and little P4O10. The HOP0 and HOP02 molecules were observed as hydrolysis intermediates. Thermolysis of P4Olo at 900-950 "C produced a nonrandomized distribution of depolymerization products, which favored P409 and PO2, and to a lesser extent P4O8. P205, and P2O4.Reactive laser ablation of red phosphorus and oxygen produced the same species as the depolymerization reactions of orthorhombic P4010 and earlier reactions of P2 with O2 and 03.57 The reaction was confined to the plume produced by the laser owing to the production of excited phosphorus fragment species. This work shows that POz is a particularly stable gaseous species20 and suggests that molecular P205 can also be observed in the gas phase.Laser ablation has been used for a number of years with great success in combination with matrix-isolation fluorescence spectroscopy by Bondybey and co-workers2I and for matrix-isolation ESR studies by Knight." This work exploits the combination of laser ablation of polymerized substrates and reactive laser ablation with matrix infrared spectroscopy. (20) Kawaguchi, K.; Saito, S.; Hirota, E.; Ohashi, N.Gas-phase infrared spectra and liquid-phase Raman spectra are presented for 1 -chlorocyclobutene and 1 -fluorocyclobutene. Complete assignments of vibrational fundamentals are proposed for these molecules of C, symmetry. For 1 -chlorocyclobutene (cm-I): (a') . Frequencies of CH-rich modes correlate closely for these isotopomer-like molecules. These results along with a Raman spectrum of liquid cyclobutene near -100 OC have provided evidence for assigning five unsettled vibrational fundamentals of cyclobutene. These modes...

Ab Initio Multiple Spawning: Photochemistry from First Principles Quantum Molecular Dynamics

“…We have analyzed the character of the normal modes and must agree with Negri and co-workers 152 that the assignment 150,151 on which Mathies 147 based his interpretation is incorrect. Although there is some component of disrotatory motion in the 1075 cm -1 normal mode, the mode is dominantly a CH 2 wag.…”

“…147 Intensity was observed in an overtone of the 1075 cm -1 (ν 15 ) normal mode at 2150 cm -1 , implying that the corresponding motion plays a significant role in the excited state dynamics on a time scale shorter than that of the resonance Raman experiment (≈50 fs). Earlier work 150,151 indicated that the ν 15 normal mode (b 1 symmetry) is of disrotatory character. (Note that the disrotatory and conrotatory modes are not normal modes.)…”

Diels–Alder Reactivities of Strained and Unstrained Cycloalkenes with Normal and Inverse-Electron-Demand Dienes: Activation Barriers and Distortion/Interaction Analysis

“…This trend is consistent with the Diels−Alder reactivities of different alkenes discussed in this contribution. The increase in distortion energy of dihedral angles indicates an increase of out-of-plane bending force constants from cyclopropene to cyclohexene, which is confirmed by the C−H out-of-plane vibration frequency observed in IR spectra of cycloalkenes (γ-CH out-of-plane bend of cycloalkenes 5−8 are 570, 33 635, 34 695, 35 and 718 cm −1 , 36 respectively). This trend in bending force constants is in accord with the change of hybridization states of the olefinic carbons suggested by the natural bond orbital (NBO) analysis.…”

Direct Observation of Disrotatory Ring-Opening in Photoexcited Cyclobutene Using ab Initio Molecular Dynamics

“…For the case of cyclohexadiene, this prediction (conrotatory) has been experimentally confirmed using both stereochemical analysis of the products in alkyl-substituted reactants − and resonance Raman spectroscopy. , Somewhat surprisingly, the results for the smaller cyclobutene (CB) molecule are less conclusive. Alkyl substitutions of CB have produced mixtures of allowed (disrotatory) and forbidden (conrotatory) photoproducts and, because of controversies in the assignment of some bands, , the interpretation of resonance Raman experiments , (which provide a more direct probe of the excited-state dynamics) has been questioned . On the basis of the original assignment of the vibrational force field of CB, and a later assignment by Craig and co-workers, the resonance Raman spectrum 13 has been interpreted to show traces of a disrotatory ring opening. , However, on the basis of their own assignment of the vibrational force field and on their interpretation of the assignment of Wiberg and Rosenberg, Negri and co-workers 15 concluded that the resonance Raman spectrum does not show “any positive hint of the activity of distrotatory ring opening motion.”…”