A large number of reactions of the type R• + HX and R• + X2 have been reported as having negative activation energies (X = I, Br, Cl). These reactions have none of the behavior of reactions that are expected to have negative activation energies. It is shown that they must be simple metathesis reactions having a single transition state, (R·H·Ẋ)⧧ or (R·X·Ẋ)⧧. It is concluded that the negative activation energies must be artifacts of the experimental techniques employed. Some of what appear to be simple metathesis reactions but which proceed via atom + radical recombination have had rate constants reported, close to the collision limit. When examined from a collisional point of view, it is shown that they require collision diameters from 8 to 25 Å, far in excess of any known long-range interaction at these distances between neutral species. Again, artifacts of the experimental methods may be responsible.

6377This consideration suggests that a search of the low-frequency UVRR spectrum might yield evidence for C-N twisting overtone enhancement. ConclusionsUVRR spectroscopy of N M A shows the amide bond to be strongly affected by the electron-acceptor (or H-bond donor) properties of its environment, in both the ground and first A-A* excited states. The observed affects can be understood as resulting from stabilization of the C -0 A* fragment orbital by electronacceptor interactions, and the consequent introduction of C-0 antibonding and C-N bonding A character into the HOMO. The result is a frequency decrease for the amide I C-0 stretching mode which is linearly dependent on the solvent acceptor number, and a frequency increase in the amide I1 and 111 modes which share the C-N stretching coordinate. H-bond acceptor interactions at the N-H bond have no affect on the amide frequencies but decrease the N-H stretching frequency in proportion to the solvent donor number. Apparently, the N-H and C=O H-bond interactions are effectively insulated from one another by the A-u separation. This insulation makes it possible to evaluate the energetics of C=O and N-H H-bonds independently from spectroscopic data.The C=O A+ stabilization by electron-acceptor interactions also introduces C-N antibonding character into the first vacant A* molecular orbital, while retaining C-0 antibonding character. Consequently, the excited-state potential is displaced more along the C-N stretching coordinate and less along the C-0 stretching coordinate. with the result that amide I1 and 111 are intensified, as is amide S via its coupling to amide 111, while amide I is weakened. Indeed, in water, amide I shows no resonance enhancement from the first A* excited state but only from the second, which is mainly a C -0 localized state.The first A* excited state is also expected to distort by twisting about the C-N bond and to have a minimum energy at a 90' twist angle, thereby providing a pathway for the photoisomerization which is readily observed for NMA via UVRR spectroscopy. This effect should be accentuated by H-bond donor interactions, due to the same orbital composition changes, as is evident from the increasing photoisomerization yield with increasing water content of water/acetonitrile mixtures. The photoisomerization yield decreases with increasing steric bulk of the C and N substituents and is expected to be negligible for polypeptides, except for terminal glycine residues.While the twisting distortion in the excited state should enhance even quantum transitions of twisting modes in the UVRR spectra, the amide V N-H out-of-plane overtone is undetectable even directly on resonance with the first A* transition. The likelist explanation is that the change in the C-N torsional force constant is minimized by a barrier to twisting in the excited state and also that the extent of C-N twisting in this mode is relatively small. Low-frequency modes involving out-of-plane displacements of the carbon substituents may be better candidates for overtone e...

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The rates of the reactions of ethyl radicals with HBr (k 7) and with Br atoms (k 8) have been measured in the temperature range 228−368 K at millitorr pressures using the very low pressure reactor (VLPR) technique. The Arrhenius function for the H atom abstraction reaction is found to be k 7 = (1.43 ± 0.06) × 10-12 exp[−(444 ± 26)/RT] cm3/(molecule s), while the ethyl radical disproportionation with Br atom shows no temperature dependence. Its average value over the entire temperature range is k 8 = (1.18 ± 0.05) × 10-11 cm3/(molecule s). Reaction 7 is significantly slower than has been reported in the only other two direct measurements, both finding a negative activation energy for k 7 of from −1.0 to −1.1 kcal/mol. The small positive activation energy found in this work for k 7 fits standard models for H atom metathesis. Combination with all known kinetic information for ethane bromination gives an average reaction enthalpy of ΔH°7 = 13.0 ± 0.2 kcal/mol using both the second- and third-law thermochemical calculations. It sets the heat of ethyl radical formation to Δf H°(C2H5) = 28.40 ± 0.25 kcal/mol and the bond dissociation enthalpy, DH°(C2H5−H) = 100.5 ± 0.3 kcal/mol.

A new, improved variant of the very low pressure reactor (VLPR) system with interchangeable discharge orifices was used for studying the compatibility of the chemical and physical processes occurring simultaneously. It is shown that the ratio of calculated and effective escape rate constants is a complex function of the reactor cell geometry for reactions of Eon-spherical symmetry.The test reaction of atomic chlorine with methane proved t o be a pure chemical process free of side reactions and was used to calibrate the system. The measured rat,e constant is k , =~ 10.993 f 0.013) x 10~':' cm'/molec-s at 25°C. ' 4 new procedure is outlined for measuring the equilibrium coostant by changing the concentrations of all three components resulting in a remarkable accuracy of K , = 1.406 t 0.034 for t,he test reaction a t 25°C.