The potential energy surfaces of methanol clusters, (CH3OH)n, n = 2-12, have been studied using density functional theory at the B3LYP/6-31G(d) and higher levels of theory. Cyclic clusters in which n methanol molecules are joined in a ring structure formed by n hydrogen bonds are shown to be more stable than structures of the same number of methanol molecules where one or more methanol molecules are outside the ring and are hydrogen-bonded to oxygens of methanols in rings of n - 1, n - 2, and so forth. So-called chain structures are generally even less stable. Furthermore, the hydrogen-bonding energy per methanol molecule of the n-ring clusters is shown to converge to an asymptotic value of about 27 kJ/mol at B3LYP/6-311+G(d,p)//B3LYP/6-31G(d) after five to six methanols are included in the cluster. As expected, there are many minima on the potential energy surfaces of the methanol clusters, the number increasing rapidly with n. A cyclic cluster of five to six methanol molecules appears to be sufficient to mimic liquid behavior as far as vibrational frequencies are concerned.

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The structures and reactivities of the alkoxy radicals methoxy (CH 3 O·), ethoxy (CH 3 CH 2 O·), 1-propoxy (CH 3 CH 2 CH 2 O·), 2-propoxy ((CH 3 ) 2 CHO·), 2-butoxy (CH 3 CH 2 CH(CH 3 )O·), tert-butoxy ((CH 3 ) 3 CO·), prop-2-enoxy (CH 2 =CHCH 2 O·), and but-3-en-2-oxy (CH 2 =CHCH(CH 3 )O·) have been investigated at the B3-LYP/6-31G(d) and CBS-RAD levels of theory. Enthalpies of formation (∆ f H 298 o ) were calculated with CBS-RAD for all the alkoxy radicals, the carbonyl and radical products of β-scission reactions, and the transition structures leading to them. The mean absolute deviation between the predicted and available experimental ∆ f H 298 o values is 5.4 kJ mol -1 . Eyring (∆H 0 ‡ , ∆H 298 ‡ , ∆G 298 ‡ ) and Arrhenius (log A, E a ) activation parameters for both the forward (β-scission) and reverse (radical addition to carbonyl) pathways were calculated. Agreement with available experimental data is very good, generally within 1-5 kJ mol -1 for E a , and 0.5 for log A. The transition structures are found to be substantially polarized, with the departing radical slightly positive, the O atom negative, and the rest of the molecule positive. The barriers for the β-scission reactions decrease with decreasing endothermicity and with decreasing ionization energy of the departing radical.Résumé : Faisant appel à des calculs théoriques aux niveaux B3-LYP/6-31G(d) et CBS-RAD de la théorie, on a étudié les structures et les réactivités des radicaux alkoxy, méthoxy (CH 3 O·), éthoxy (CH 3 CH 2 O·), 1-propoxy (CH 3 CH 2 CH 2 O·), 2-propoxy [(CH 3 ) 2 CHO·], 2-butoxy [CH 3 CH 2 CH(CH 3 )O·], tert-butoxy [(CH 3 ) 3 CO·], prop-2-énoxy (CH 2 =CHCH 2 O·) et but-3-én-2-oxy [CH 2 =CHCH(CH 3 )O·]. On a calculé les enthalpies de formation, ∆ f H 298 o , au niveau CBS-RAD de la théorie pour tous les radicaux alkoxy, tous les produits carbonylés et tous les produits radicalaires provenant de réac-tions de β-scission et toutes les structures de transition qui y conduisent. La déviation absolue moyenne entre les valeurs prédites et les valeurs de ∆ f H 298 o expérimentales disponibles est de 5,4 kJ mol -1 . On a aussi calculé les paramètres d'activation d'Eyring (∆H 0 ‡ , ∆H 298 ‡ , ∆G 298 ‡ ) et d'Arrhenius (log A, E a ) pour la réaction vers la droite (β-scission) et pour la réaction inverse (addition d'un radical sur le carbonyle). L'accord entre les valeurs calculées et les valeurs expéri-mentales disponibles est bon, généralement entre 1 et 5 kJ mol -1 pour les valeurs de E a et de 0,5 pour le log A. On a trouvé que les structures de transition sont assez polarisées alors que le radical qui se détache est légèrement positif, l'atome d'oxygène est négatif et que le reste de la molécule est positif. Les barrières aux réactions de β-scission diminuent avec une augmentation du caractère endothermique et avec une diminution de l'énergie d'ionisation du radical qui se détache.

Ab initio molecular orbital calculations indicate that the bond dissociation energies (BDE) for homolytic cleavage of CX bonds (X = C, N, 0, F) are increased by protonation of the corresponding alkyl, amine, alcohol, or fluoride functional groups; the effect of deprotonation of these groups is rather small for saturated species, whereas for unsaturated ones deprotonation leads to large increases in the CX BDEs. The effects on the CC BDEs in CCX compounds are quite systematic: protonation of X increases the CC BDE, while the converse holds for deprotonation. Two types of correlation between bond lengths and homolytic bond dissociation energies are observed. Firstly, protonation and deprotonation lead to a normal correlation for the adjacent CC bonds: the bond length decreases as the BDE increases. Protonation, however, results in an anomalous correlation for the CX bonds: the bond length increases as the BDE increases. These observations stabilization, and competing heterolytic dissociation.