Conformational energies for 2-substituted butanes

The conformational free energies for some 2-substituted butanes where X = F, Cl, CN, and CCH were calculated using G3-B3, CBS-QB3, and CCSD(T)/6-311++G(2d,p) as well as other theoretical levels. The above methods gave consistent results with free energies relative to the trans conformers as follows: X = CCH, g+ = 0.77 +/- 0.05 kcal/mol. g- = 0.88 +/- 0.05 kcal/mol; X = CN, g+ = 0.85 +/- 0.05 kcal/mol, g- = 0.75 +/- 0.05 kcal/mol; X = Cl, g+ = 0.70 +/- 0.05 kcal/ml, g- = 0.80 +/- 0.05 kcal/mol; and X = F, g+ = 0.53 +/- 0.05 kcal/mol, g- = 0.83 +/- 0.05 kcal/mol. The conformational free energies also were estimated using the observed liquid phase IR spectra and intensities calculated using B3LYP/6-311++G** and MP2/6-311++G**. The rotational free energy profiles for all of the compounds were estimated at the G3-B3 level.     

1,5] Sigmatropic hydrogen shifts in cyclic 1,3-dienes

Density functional calculations have been carried out for [1,5] hydrogen shifts in 1,3-cycloalkadienes (cyclohexadiene, cycloheptadiene, and cyclooctadiene). The complexity of the potential surfaces of these reactions was found to increase with ring size. For 1,3-cyclohexadiene a single transition structure for the [1,5] hydrogen shift was located, which connects the two enantiomeric conformers. For 1,3-cycloheptadiene two enantiomeric transition structures for the [1,5] hydrogen shift were located, which interconnect three conformers of the diene, a pair of enantiomeric conformers and a third achiral conformer. Finally for 1,3-cyclooctadiene two diastereomeric transition structures were found in addition to six conformers (three pairs of enantiomeric conformers) of the diene. Calculated activation energies for the [1,5] hydrogen shifts were found to be in qualitative agreement with experiment. Variation in these energies are attributed to strain energies present in either the diene or the transition structure.     

Theoretical predictions of the initial decomposition steps of dimethylnitramine

The structures and energies of the reactants, products, and transition states of the initial steps in the gas-phase decomposition of dimethylnitramine (DMNA) have been determined by quantum chemical calculations at the B3LYP density-functional theory, MP2, and G2 levels. The pathways considered are NO2 elimination, HONO elimination, and nitro-nitrite rearrangement. The NO2 elimination is predicted to be the main channel of the gas-phase decomposition of DMNA in accord with experiment. The values of the Arrhenius parameters, log A=16.6+/-0.5 and Ea=40.0+/-0.6 kcal/mol, for the N-NO2 bond-fission reaction were obtained using a canonical variational theory with B3LYP energies and frequencies. The HONO-elimination channel has the next lowest activation energy of 44.7+/-0.5 kcal/mol (log A=13.6+/-0.5) and is characterized by a five-member transition-state configuration in which a hydrogen atom from one of the methyl groups is transferred to an oxygen atom of NO2. Tunneling contributions to the rate of this reaction have been estimated. The nitro-nitrite rearrangement reaction occurs via a transition state in which both oxygen atoms of NO2 are loosely bound to the central nitrogen atom, for which Rice-Ramsperger-Kassel-Marcus theory predicts log A=14.4+/-0.6 and Ea=54.1+/-0.8 kcal/mol.     

Dynamic processes in [16]annulene: Möbius bond-shifting routes to configuration change

Density functional and ab initio methods have been used to study the mechanisms for key dynamic processes of the experimentally known S4-symmetric [16]annulene (1a). Using BH&HLYP/6-311+G** and B3LYP/6-311+G**, we located two viable stepwise pathways with computed energy barriers (Ea = 8-10 kcal/mol) for conformational automerization of 1a, in agreement with experimental data. The transition states connecting these conformational minima have M?bius topology and serve as starting points for non-degenerate pi-bond shifting (configuration change) via M?bius aromatic transition states. The key transition state, TS1-2, that connects the two isomers of [16]annulene (CTCTCTCT, 1 --> CTCTTCTT, 2) has an energy, relative to the S4 isomer, that ranged from 6.9 kcal/mol (B3LYP/6-311+G**) to 16.7 kcal/mol (BH&HLYP/6-311+G**), bracketing the experimental barrier. At our best level of theory, CCSD(T)/cc-pVDZ(est), this barrier is 13.7 kcal/mol. Several other M?bius bond-shifting transition states, as well as M?bius topology conformational minima, were found with BH&HLYP energies within 22 kcal/mol of 1a, indicating that many possibilities exist for facile thermal configuration change in [16]annulene. This bond-shifting mechanism and the corresponding low barriers contrast sharply with those observed for cis/trans isomerization in acyclic polyenes, which occurs via singlet diradical transition states. All M?bius bond-shifting transition states located in [16]- and [12]annulene were found to have RHF --> UHF instabilities with the BH&HLYP method but not with B3LYP. This result appears to be an artifact of the BH&HLYP method. These findings support the idea that facile thermal configuration change in [4n]annulenes can be accounted for by mechanisms involving twist-coupled bond shifting.     

Gaseous arginine conformers and their unique intramolecular interactions

Extensive ab initio calculations were employed to characterize stable conformers of gaseous arginine, both the canonical and zwitterionic tautomers. Step-by-step geometry optimizations of possible single-bond rotamers at the B3LYP/6-31G(d), B3LYP/6-31++G(d,p), and MP2/6-31++G(d,p) levels yield numerous structures that are more stable than any known ones. The final electronic energies of the conformers were determined at the CCSD/6-31++G(d,p) level. The lowest energies of the canonical and zwitterionic structures are lower than the existing values by 2.0 and 2.3 kcal/mol, respectively. The relative energies, rotational constants, dipole moments, and harmonic frequencies of the stable conformers remain for future experimental verification. The conformational distributions at various temperatures, estimated according to thermodynamic principles, consist almost exclusively of the newly found structures. One striking feature is the occurrence of blue-shifting hydrogen bonds in all six of the most stable conformers. A unique feature of important conformations is the coexistence of dihydrogen and blue- and red-shifting hydrogen bonds. In addition to the hydrogen bonds, the stereoelectronic effects were also found to be important stabilization factors. The calculated and measured proton affinities agree within the theoretical and experimental uncertainties, affirming the high quality of our conformational search. The theoretical gas-phase basicity of 245.9 kcal/mol is also in good agreement with the experimental value of 240.6 kcal/mol. The extensive searches establish firmly that gaseous arginine exists primarily in the canonical and not the zwitterionic form.     

The amide rotational barriers in picolinamide and nicotinamide: NMR and ab initio studies

Pyridine carboxamides are a class of medicinal agents with activity that includes the reduction of iron-induced renal damage, the regulation of nicotinamidase activity, and radio- and chemosensitization. Such pharmacological activities, and the prevalence of the carboxamide moiety and the importance of amide rotations in biology, motivate detailed investigation of energetics in these systems. In this study, we report the use of dynamic nuclear magnetic resonance to measure the amide rotational barriers in the pyridine carboxamides picolinamide and nicotinamide. The activation enthalpies and entropies of DeltaH++ = 12.9 +/- 0.3 kcal/mol and DeltaS++ = -7.7 +/- 0.9 cal/mol K for nicotinamide and DeltaH++ = 18.3 +/- 0.4 kcal/mol and DeltaS++ = +1.3 +/- 1.0 cal/mol K for picolinamide report a substantial energetic difference for these regioisomers. Ab initio calculations of the rotational barriers are in good agreement with the experimentally determined values and help partition the 5.4 kcal/mol enthalpy difference into its major contributions. Of principal importance are the variations in steric interactions in the ground states of picolinamide and nicotinamide, superior pi electron donation from the pyridine ring in the transition state of nicotinamide, and an intramolecular hydrogen bond in the ground state of picolinamide.     

Nonclassical Aryl Radicals: Intermediates or Transition States for the Hydrogen Shift Reactions?

Electronic properties of aryl radicals obtained by removing single hydrogen atoms from the sterically congested regions of benzo[c]phenanthrene, biphenyl, triphenylene, phenanthrene, and perylene are studied at the UBLYP/6-311G level of theory. Two structures are considered by each radical, the classical one involving a C-H.C arrangement of atoms and the nonclassical one possessing a three-center C-H-C linkage. The five nonclassical radicals under study are found to be transition states for degenerate 1,4- and 1,5-hydrogen shift reactions that interconvert the classical species. However, the results of the present calculations indicate that the nonclassical structures with the C-H distances in the C-H-C linkages shorter than 1.34 ? should be energy minima representing potentially observable chemical systems. The predicted energy barrier to the 1,5-hydrogen shift in the 1-benzo[c]phenanthrenyl radical is only 9.3 kcal/mol (6.1 kcal/mol with the zero-point energies included), making the hydrogen migration in this system facile at relatively low temperatures. Rigorous analysis of the computed electronic wave functions provides a clear-cut picture of bonding in both the classical and nonclassical aryl radicals.     

13C NMR, infrared, solvation and theoretical investigation of the conformational isomerism in 1-haloacetones (X = Cl, Br and I)

The solvent dependence of the 13C NMR spectra of chloroacetone (CA), bromoacetone (BA) and iodoacetone (IA) are reported and the 3J(CH) couplings analysed using ab initio calculations and solvation theory. In CA the energy difference (E(cis) - E(gauche)) between the cis (Cl-C-C=O 0 degrees) and gauche (Cl-C-C=O 155 degrees) conformers is 1.7 kcal mol(-1) in the vapour, decreasing to 0.8 kcal mol(-1) in CCl4 solution and to -1.0 kcal mol(-1) in the pure liquid. The conformational equilibrium, in BA, is between the more polar cis (Br-C-C=O 0 degrees) and gauche (Br-C-C=O 132 degrees) conformations. The energy difference (E(cis) - E(gauche)) is 1.8 kcal mol(-1) in the vapour, decreasing to 0.9 kcal mol(-1) in CCl4 solution and to -0.4 kcal mol(-1) in the pure liquid. The energy difference (E(cis) - E(gauche)), in IA, between the cis (I-C-C=O 0 degrees) and gauche (I-C-C=O 104 degrees) conformers is 1.1 kcal mol(-1) in the vapour phase, decreasing to 0.5 kcal mol(-1) in CCl4 solution and to -0.5 kcal mol(-1) in the pure liquid. The vapour state energy difference for BA [1.4 kcal mol(-1) at B3LYP/6-311++G(d,p)] and for IA [1.6 kcal mol(-1) at B3LYP/6-311++G(d,p)/LANL2DZ)] are in very good agreement with the above values. For CA the agreement is also satisfactory [1.4 kcal mol(-1) at B3LYP/6-311++G(d,p)].     

Theoretical prediction of the heats of formation of C2H5O* radicals derived from ethanol and of the kinetics of beta-C-C scission in the ethoxy radical

Thermochemical parameters of three C(2)H(5)O* radicals derived from ethanol were reevaluated using coupled-cluster theory CCSD(T) calculations, with the aug-cc-pVnZ (n = D, T, Q) basis sets, that allow the CC energies to be extrapolated at the CBS limit. Theoretical results obtained for methanol and two CH(3)O* radicals were found to agree within +/-0.5 kcal/mol with the experiment values. A set of consistent values was determined for ethanol and its radicals: (a) heats of formation (298 K) DeltaHf(C(2)H(5)OH) = -56.4 +/- 0.8 kcal/mol (exptl: -56.21 +/- 0.12 kcal/mol), DeltaHf(CH(3)C*HOH) = -13.1 +/- 0.8 kcal/mol, DeltaHf(C*H(2)CH(2)OH) = -6.2 +/- 0.8 kcal/mol, and DeltaHf(CH(3)CH(2)O*) = -2.7 +/- 0.8 kcal/mol; (b) bond dissociation energies (BDEs) of ethanol (0 K) BDE(CH(3)CHOH-H) = 93.9 +/- 0.8 kcal/mol, BDE(CH(2)CH(2)OH-H) = 100.6 +/- 0.8 kcal/mol, and BDE(CH(3)CH(2)O-H) = 104.5 +/- 0.8 kcal/mol. The present results support the experimental ionization energies and electron affinities of the radicals, and appearance energy of (CH(3)CHOH+) cation. Beta-C-C bond scission in the ethoxy radical, CH(3)CH2O*, leading to the formation of C*H3 and CH(2)=O, is characterized by a C-C bond energy of 9.6 kcal/mol at 0 K, a zero-point-corrected energy barrier of E0++ = 17.2 kcal/mol, an activation energy of Ea = 18.0 kcal/mol and a high-pressure thermal rate coefficient of k(infinity)(298 K) = 3.9 s(-1), including a tunneling correction. The latter value is in excellent agreement with the value of 5.2 s(-1) from the most recent experimental kinetic data. Using RRKM theory, we obtain a general rate expression of k(T,p) = 1.26 x 10(9)p(0.793) exp(-15.5/RT) s(-1) in the temperature range (T) from 198 to 1998 K and pressure range (p) from 0.1 to 8360.1 Torr with N2 as the collision partners, where k(298 K, 760 Torr) = 2.7 s(-1), without tunneling and k = 3.2 s(-1) with the tunneling correction. Evidence is provided that heavy atom tunneling can play a role in the rate constant for beta-C-C bond scission in alkoxy radicals.     

Cheletropic decomposition of cyclic nitrosoamines revisited: the nature of the transition states and a critical role of the ring strain

The cheletropic decompositions of 1-nitrosoaziridine (1), 1-nitroso-Delta(3)-pyrroline (2), 7-nitroso-7-azabicyclo[2.2. 1]hepta-2,5-diene (3), and 6-nitroso-6-azabicyclo[2.1.1]hexa-4-ene (4) have been studied theoretically using high level ab initio computations. Activation parameters of the decomposition of nitrosoaziridine 1 were obtained experimentally in heptane (DeltaH()(298) = 18.6 kcal mol(-)(1), DeltaS()(298) = -7.6 cal mol(-)(1) K(-)(1)) and methanol (20.3 kcal mol(-)(1), 0.3 cal mol(-)(1) K(-)(1)). Among employed theoretical methods (B3LYP, MP2, CCD, CCSD(T)//CCD), the B3LYP method in conjunction with 6-31+G, 6-311+G, and 6-311++G(3df,2pd) basis sets gives the best agreement with experimental data. It was found that typical N-nitrosoheterocycles 2-4 which have high N-N bond rotation barriers (>16 kcal mol(-)(1)) extrude nitrous oxide via a highly asynchronous transition state with a planar ring nitrogen atom. Nitrosoaziridine 1, with a low rotation barrier (<9 kcal mol(-)(1)) represents a special case. This compound can eliminate N(2)O via a low energy linear synperiplanar transition state (DeltaH()(298) = 20.6 kcal mol(-)(1), DeltaS()(298) = 2.5 cal mol(-)(1) K(-)(1)). Two higher energy transition states are also available. The B3LYP activation barriers of the cheletropic fragmentation of nitrosoheterocycles 2-4 decrease in the series: 2 (58 kcal mol(-)(1)) > 3 (18 kcal mol(-)(1)) > 4 (12) kcal mol(-)(1). The relative strain energies increase in the same order: 2 (0 kcal mol(-)(1)) < 3 (39 kcal mol(-)(1)) < 4 (52 kcal mol(-)(1)). Comparison of the relative energies of 2-4 and their transition states on a common scale where the energy of nitrosopyrroline 2 is assumed as reference indicates that the thermal stability of the cyclic nitrosoamines toward cheletropic decomposition is almost entirely determined by the ring strain.     

Gas-phase kinetics and activation parameters for thermal 1,5-hydrogen shifts interconverting monodeuterium-labeled 1,3-cyclohexadienes

Thermal equilibrations among the three possible monodeuterium-labeled 1,3-cyclohexadienes have been followed in the gas phase at temperatures from 254 to 284 degrees C. The temperature-dependent rate constants for the 1,5-shift of a single hydrogen lead to the activation parameters E(a) = (40.1 +/- 0.8) kcal/mol, log A = (12.1 +/- 0.3), and DeltaS = -(6.3 +/- 1.3) e.u. These activation parameters are reconciled with experimental values reported earlier for reactions starting with 1,4-d(2)-cyclohexadiene.     

Syntheses, conformations, and basicities of bicyclic triamines

The multistep syntheses of several bicyclic triamines are described, all of which have an imbedded 1,5,9-triazacyclododecane ring. In 1,5,9-triazabicyclo[7.3.3]pentadecanes 12, 13, 15, and 16, two nitrogens are bridged by three carbons. The monoprotonated forms of these triamines are highly stabilized by a hydrogen-bonded network involving the bridge and both bridgehead nitrogens, producing a difference of more than 8 pK(a) units in acidities of their monoprotonated and diprotonated forms. The one- and zero-carbon bridges in 1,5,9-triazabicyclo[9.1.1]tridecane (23) and 7-methyl-1,5,9-triazabicyclo[5.5.0]dodecane (39) do not enhance the stabilities of their monoprotonated forms. X-ray crystal structures and computational studies of 12.HI and 16.HI reveal similar, but somewhat weaker, hydrogen-bonded networks, relative to 15.HI. The activation free energies for conformational inversion of 13.HI (14.4 +/- 0.2 kcal/mol), 16.HI (15.0 +/- 0.1 kcal/mol) and 16 (8.8 +/- 0.3 kcal/mol) were measured by variable-temperature (1)H and (13)C NMR spectroscopy. These experimental barriers give an estimate of 6.2 kcal/mol for the strength of the bifurcated hydrogen bond between the bridge nitrogen and cavity proton in 16.HI. Computational studies support the hypothesis that N-inversion occurs in an open conformation, leading to an estimate of 10.32 kcal/mol for the enthalpy of the bifurcated hydrogen bond in 16.HI in the gas phase.     

Just a near attack conformer for catalysis (chorismate to prephenate rearrangements in water,antibody, enzymes,and their mutants)

Standard free energies for formation of ground-state reactive conformers (DeltaGN degrees ) and transition states (DeltaG) in the conversion of chorismate to prephenate in water, B. subtilis mutase, E. coli mutase, and their mutants, as well as a catalytic antibody, are related by DeltaG = DeltaGN degrees + 16 kcal/mol. Thus, the differences in the rate constants for the water reaction and catalysts reactions reside in the mole fraction of substrate present as reactive conformers (NACs). These results, and knowledge of the importance of transition state stabilization in other cases, suggest a proposal that enzymes utilize both NAC and transition state stabilization in the mix required for the most efficient catalysis.     

Thermal decomposition of a series of 1,2-diazetines

A homologous series of tricyclic diazetines (6a-c), differing by the number of methylene groups in the saturated bridges of the fused carbon bicycles, was synthesized. The DeltaH++ of decomposition for each of the diazetines to afford N2 and the corresponding alkene was determined experimentally: 6a, 31.7; 6b, 39.3; 6c, 38.8 kcal/mol. The ground-state strain energy of each diazetine was estimated utilizing computationally obtained DeltaHf's for each of the experimentally investigated diazetines as well as several other diazetines whose DeltaH++'s had been previously reported in the literature. The sum of the ground-state strain energies and DeltaH++'s of decomposition for all of the diazetines was nearly constant, with an average value of 59 kcal/mol, suggesting that all of the diazetines decompose via the same mechanism. Generally, the higher the ground-state strain energy of the diazetine, the less the DeltaH++ for decomposition. The decomposition transition states for 6a-c and 7 were modeled computationally at the RB3LYP/6-311+G(3df,2p)//UB3LYP/6-31+G(d,p) level. The agreement of the experimentally determined DeltaH++ values with transition-state energies obtained computationally supports the reaction mechanism originally proposed by Yamabe that the elimination process occurs by an unsymmetrical, yet concerted, transition state with strong biradical character.     

Calculation of relative energies of permethylated oligosilane conformers in vapor and in alkane solution

The geometries of 35 conformers of Me(SiMe2)nMe (n = 4, 1; n = 5, 2; n = 6, 3; n = 7, 4) were optimized at the MP2/VTDZ level, and CCSD(T) single-point calculations were done at three MP2/VTDZ conformer geometries of 1. The relative ground-state energies of the conformers of 1-4 in the gas phase were obtained from the MP2/VTDZ electronic energy, zero-point vibrational energy, and thermal corrections at 0, 77, and 298 K. Relative energies in an alkane solvent at 77 and 298 K were obtained by the addition of solvation energies, obtained from the SM5.42R model. The calculated energies of 26 of the conformers (n = 4-6) have been least-squares fitted to a set of 15 additive increments associated with each Si-Si bond conformation and each pair of adjacent bond conformations, with mean deviations of 0.06-0.20 kcal/mol. An even better fit for the energies of 24 conformers (mean deviations, 0.01-0.09 kcal/mol) has been obtained with a larger set of 19 increments, which also contained contributions from selected combinations of conformations of three adjacent bonds. The utility of the additive increments for the prediction of relative conformer energies in the gas phase and in solution has been tested on the remaining nine conformers (n = 6, 7). With the improved increment set, the average deviation from the SM5.42R//MP2 results for solvated conformers at 298 K was 0.18 kcal/mol, and the maximum error was 0.98 kcal/mol.     

Gas-phase kinetics and activation parameters for thermal [1,5] hydrogen shifts interconverting monodeuterium- labeled 1,3-cycloheptadienes

The kinetics of thermal equilibrations among monodeuterium-labeled 1,3-cycloheptadienes in the gas phase followed from 154 to 190 degrees C provide activation parameters for the [1,5] shift of a single hydrogen: E(a) = (27.5 +/- 0.9) kcal/mol and log A = 9.7 [corrected] +/- 0.4. These activation parameters imply a comparatively low E(a) barrier balanced by demandingly specific geometric constraints, for DeltaS (170 degrees C) = -17 [corrected] e.u.     

3,3-Sigmatropic Shifts of Allyl Group Along Cyclopentadiene Ring Perimeter

Reversible non-degenerate 3,3-sigmatropic shifts of the allyl group along the perimeter of the five-membered ring occurring with energy barriers ΔG°≠ = 28.5–30.2 kcal/mol (o-dichlorobenzene-d4) have been detected in the allyl derivatives of 5-methyl-1,2,3,4-tetramethoxycarbonylcyclopentadiene by NMR method. Using DFT B3LYP/6-311++G(d,p) method, it has been shown that degenerate migrations of the allyl group in the related 5-allyl-1,2,3,4,5-pentamethoxycarbonylcyclopentadiene should occur via 3,3-sigmatropic shift through transition states with conformation of a six-membered ring (chair or boat, with close barriers ΔG°≠ = 27.4 or 27.7 kcal/mol, respectively). The simulated higher barrier of alternative 1,5-sigmatropic shifts of the allyl group (ΔG°≠ = 30.8 kcal/mol) indicates the energy preference of the migrations via 3,3-shifts.     

Structure and fluxional behavior of pentamethoxycarbonylcyclopentadiene

Quantum-chemical calculations by the density functional theory method at the B3LYP/6- 311++G** level have shown that the fulvene form of pentamethoxycarbonylcyclopentadiene 1 in the gas phase is more favorable in energy than cyclopentadiene form 2 by ΔE_ZPE = 7.8 kcal/mol. The fluxional behavior of fulvene 1, detected by dynamic NMR can be explained by the mechanism of circular low-barrier 1,9-O,O'-H shifts accompanied by rotations of the hydroxymethoxymethylene substituent about the С=С bond with the activation barrier ΔE_ZPE= 23.5 (gas) and 20.9 (CH₂Cl₂) kcal/mol. This reaction path is 18.6 kcal/mol more favorable in energy than transition of fulvene 1 to cyclopentadiene 2 with subsequent 1,5- sigmatropic hydrogen shifts in the five-membered ring.     

A new scheme for determining the intramolecular seven-membered ring N-H...O=C hydrogen-bonding energies of glycine and alanine peptides

In this paper a new scheme was proposed to calculate the intramolecular hydrogen-bonding energies in peptides and was applied to calculate the intramolecular seven-membered ring N-H...O=C hydrogen-bonding energies of the glycine and alanine peptides. The density-functional theory B3LYP6-31G(d) and B3LYP6-311G(d,p) methods and the second-order Moller-Plesset perturbation theory MP26-31G(d) method were used to calculate the optimal geometries and frequencies of glycine and alanine peptides and related structures. MP26-311++G(d,p), MP26-311++G(3df,2p), and MP2/aug-cc-pVTZ methods were then used to evaluate the single-point energies. It was found that the B3LYP6-31G(d), MP26-31G(d), and B3LYP6-311G(d,p) methods yield almost similar structural parameters for the conformers of the glycine and alanine dipeptides. MP2/aug-cc-pVTZ predicts that the intramolecular seven-membered ring N-H...O=C hydrogen-bonding strength has a value of 5.54 kcal/mol in glycine dipeptide and 5.73 and 5.19 kcal/mol in alanine dipeptides, while the steric repulsive interactions of the seven-membered ring conformers are 4.13 kcal/mol in glycine dipeptide and 6.62 and 3.71 kcal/mol in alanine dipeptides. It was also found that MP26-311++G(3df,2p) gives as accurate intramolecular N-H...O=C hydrogen-bonding energies and steric repulsive interactions as the much more costly MP2/aug-cc-pVTZ does.     

Effect on kinetics by deuterium in the 1,5-hydrogen shift of a cisoid-locked 1,3(Z)-pentadiene, 2-methyl-10-methylenebicyclo[4.4.0]dec-1-ene: evidence for tunneling?

Prompted by extensive theoretical interest in the role of tunneling in the intramolecular 1,5-hydrogen shift in 1,3(Z)-pentadienes and the large uncertainty in the published values of the theoretically relevant kinetic deuterium-isotope effect and its dependence on temperature, we have examined a degenerate bicyclic version, 2-methyl-10-methylenebicyclo[4.4.0]dec-1-ene, which is locked into the rearrangement-competent cisoid conformation, in the hope of obtaining more precise and accurate values. From rate constants determined over a range of 33 degrees C from 167.7 to 201.6 degrees C, Arrhenius parameters, Ea = 32.8 +/- 0.4 kcal mol(-1) and log A = 11.1 +/- 0.2, were obtained. An average kinetic isotope effect of 4.2 +/- 0.5 obtained from all values for kH/kD and k-H/k-D may be compared with a value of 5.0 +/- 0.3, recalculated from data in the pioneering publication of Roth and K?nig. From a highly problematic extrapolation of the temperature dependence, a value of kH/kD of 16.6 (standard error between 6.5 and 43) is calculated for the kinetic isotope effect at 25 degrees C (Roth and K?nig: 12.2). With curvature in Arrhenius plots being one of the three types of experimental evidence considered indicative of tunneling, the kinetic study of the previously published rearrangement of 1-phenyl-5-p-tolyl-1,3(Z)-pentadiene has been extended over a period of 339 days to a range of 108 degrees C (77-185 degrees C) without discerning any deviation from a straight-line Arrhenius plot: Ea = 28.7 +/- 0.5 (kcal mol(-1)) and log A = 9.41 +/- 0.30.