J. Paul Chauvel

Gas-phase NMR studies of alcohols. Intrinsic acidities

Abstract Gas-phase (≈100 Torr) 1H NMR spectra of eighteen simple aliphatic and unsaturated alcohols, four fluorinated alcohols, and two thiols were obtained at 148.6°C where hydrogen bonding has little effect on chemical shifts. For the methanol, ethanol, n-propanol, i-propanol, t-butanol, i- butanol, neopentanol, 2,2,2-trifluoroethanol and benzyl alcohol, the observed hydroxylic proton chemical shifts correlate with previously obtained relative gas-phase acidities from thermochemical analysis which employed equilibrium constants of proton transfer reactions measured via mass spectroscopic and ion cyclotron resonance techniques. The correlational dependence is 10.3(0.5) kcal/mol ppm with a correlation coefficient of 0.99. These results demonstrate that the trend of increasing acidity with increasing size of the alkyl substituent is also reflected in the neutral forms of the alcohols, indicating that the polarizability of the ionic forms is not the only determining factor in relative gas-phase acidities of alcohols. Although factors affecting the hydroxylic proton chemical shifts of the larger substituted and unsaturated alcohols are more complex, their observed 1H NMR spectra also reflect this trend. For methanol and ethanol observed gas-phase 1H chemical shifts are also compared with recent theoritical calculations. 3JHH coupling constants across CO bonds are ≈ 5.5 Hz, significantly smaller than typical 3JHH coupling across sp3 hybrid CC bonds.

Conformational kinetics of methyl nitrite. I. NMR spectral evidence for statistical intramolecular vibrational redistribution

Pressure dependent rate constants for syn⇄anti conformational exchange in gaseous methyl nitrite and in gaseous methyl nitrite–CO2 mixtures have been obtained from line shape analyses of 1H NMR spectra. The pressure dependence of the exchange rates is consistent with a specific reaction rate constant of ∼1×109/s which agrees with RRKM calculations demonstrating that intramolecular vibrational redistribution is occurring at the statistical limit in methyl nitrite molecules with ∼12 kcal/mol of internal vibrational energy. At 12 kcal/mol methyl nitrite has a state density of ∼160/cm−1. These results indicate strong anharmonic and/or Coriolis coupling between vibrational levels. Bimolecular rate data for methyl nitrite and methyl nitrite–CO2 mixtures are consistent with a collisional efficiency βp for CO2 of 0.95(8) for activation of syn–anti conformational exchange at 258.8 K.

Conformational kinetics of methyl nitrite. II. Phase dependence of kinetic parameters

Temperature and pressure dependent exchange broadened NMR spectra of gaseous methyl nitrite are consistent with the following kinetic parameters for the syn–anti conformational interconversion: Eact (∞), 12.2(4) kcal/mol; Eact (bimolecular), 9.6(2) kcal/mol; ΔH‡, 11.7(4) kcal/mol; ΔG‡298 11.97(7) kcal/mol; and ΔS‡298 , −0.9(4) cal/mol K. Kinetic parameters for interconversion in the neat liquid and in a 1% solution in CS2 are slightly higher: Eact; 12.9(2); ΔH‡, 12.4(2) kcal/mol; ΔG‡, 11.81(8) kcal/mol; and ΔS‡, 2.6(2) cal/mol K. These remarkably small solvent effects indicate that dielectric effects on conformational dynamics in this system are nearly equal in magnitude and opposite in direction to solvent internal pressure effects.

Conformational kinetics of methyl nitrite. III. Collisional energy transfer in the bimolecular region

Bimolecular region unimolecular rate constants for the syn=anti conformed exchange in gaseous methyl nitrite and in methyl nitrite–bath gas (bath gases being He, Ne, Ar, H2, N2, O2, CO, CO2, NH3, SF6, CH4, C2H6, C3H8, n‐C4H10, and n‐C5H12) were obtained from line shape analysis of 200 MHz 1H NMR spectra. The pressure dependent rate constants were analyzed to yield relative bimolecular collisional activation efficiencies βMp on a pressure per pressure basis for each bath gas. The resulting βMp indicate that, for methyl nitrite conformational interconversion, which occurs at a low threshold energy, E0=11.70 (0.40) kcal/mol, and a low vibrational state density ρ(E0)=160 states/cm−1 intermolecular collisional energy transfer is dominated by the attractive portion of the intermolecular potential, with the energy transfer in the collisional complex being statistical. Corrections to previously published data are given.

Conformational kinetics of methyl nitrite. IV. Bimolecular kinetics at low pressures

Rate constants and associated kinetic parameters for the methyl nitrite conformational interconversion process have been determined at low pressures (∼5–70 Torr) where the kinetics are very nearly second order. Comparisons with model calculations demonstrate that vibrational redistribution is statistical and very nearly ergodic in methyl nitrite molecules possessing ∼12 kcal mol−1 internal energy.

Conformational kinetics of methyl nitrite. Temperature dependence of helium collision efficiency

Abstract The temperature dependence of the helium collision efficiency, β p He , has been determined for the methyl nitrite conformational exchange from lineshape analysis of 200 MHz 1 H gas-phase NMR spectra. An 18% decrease in β p He over a 30°C increase in temperature is observed and discussed in relation to the previously studied CH 3 NCHe system.

Gas phase NMR spectra of N,N-dimethylnitrosamine. Environmental effects on kinetic parameters

Abstract Gas phase 1 H NMR spectra of N,N -dimethylnitrosamine are consistent with first order chemical exchange rate constants which are ca. 25 times faster than those observed in neat liquids at corresponding temperatures. The associated kinetic parameters: E act (∞), 20.5(1.1) kcal mol −1 , Δ H ‡ , 19.7(1.0) kcal mol −1 and Δ G ‡ , 21.1(0.4) kcal mol −1 are approximately 2.5 kcal mol −1 lower than the most recently reported values for the neat liquid. The observed phase dependence is consistent with a process proceeding via a freely rotating transition state.