Semiempirical Study of Intramolecular Hyclrogen Bond in 6-Hydroxy-2-formylfulvene and 9-Hydroxyphenalenone

It is shown that semiempirical MINDO/3, MNDO and AMI methods are very useful in discussing intramolecular hydrogen . bonding in sizeable systems. The present results indicate that intramolecular H-bonds in 6-hydroxy-2-formylfulvene and 9-hydroxyphenalenone are asymmetric, in fuU accordance with ESCA observations and DQCC measurements. The former molecule should exhibit a stronger H-bond. On the other hand, the barrier height for proton tunnelling is higher in the latter compound. Relative stabilities of various species are interpreted by employing the energy partitioning technique

Theoretical Study of (3, 6) Cyclohemiketal Form of L-Ascorbic Acid

Sažetak The question of the stability of the bicyclic form of AA is addressed by several theoretical methods varying in the level of sophistication. Both semi- empirical and ab initio approaches indicate that the bicyclic form is somewhat more stable than the 2,3-enediol tautomer (7). However, the tautomer I has a significantly higher dipole moment indicating that it gains additionally in stability if dissolved in polar solvents due to strong electrostatic solvent-solute interactions

The Calculation of the Heats of Formation, Heats of Hydrogertation and Strain Energies in Nonconjugated Hydrocarbons by the Maximum Overlap Approximation

The heats of formation in nonconjugated hydrocarbons and the strain energies of small ring compounds were calculated within the (MOA) maximum overlap approximation. The agreement ·with · experiment is fairly good. The results indicate that the overlap l:)etween the neighbouring bonding orbitals is responsible for the greatest part in the heats of formation and that the origin of .the strain energies in small cyclic hydrocarbons is the bending of the P cc hybrids. The calculated heats of hydrogenation for some characteristic olefins are in good qualitative agreement with the · experimental values

A Semiempirical Study of Intramolecular Hydrogen Bonding in Some Naphthazarin Derivatives

Structural and energetic properties of some derivatives of naphthazarin are considered by the semiempirical AMI method. It is shown that these systems, possessing intramolecular hydrogen bonds, are qualitatively well described at the AMI level of theory. This conclusion is supported by a good qualitative agreement with available experimental data. The strength of the intramolecular hydrogen bond is briefly discussed. It appears that their energies are roughly additive

Rates of Oxidation of Some Bicyclic Alcohols with Silver Carbonate on Celite

It is well known that silver carbonate is a powerful agent for the oxidation of both primary and secondary alcohols1\u272 Recently Fetizon introduced a convenient modification of this procedure by using silver carbonate precipitated on celite3-

Theoretical Studies of the Solvent Effect on the Stability of the Ascorbic Acid Tautomers

AMI calculations on three conformers of each of the four tautomeric forms of ascorbic acid are reported. According to the results for isolated molecules, tautomer 3 should be lowest in energy, whereas experimentally only tautomer 1 is observed in the crystal as well as in solution. Solvent effects on the relative stability of the various structures are assessed by applying a combined quantum chemical and force field approach. It turned out to be very important to fully optimize both the water environment and the solute, which was achieved iteratively by means of a suitably modified AMI Hamiltonian taking into account the charge polarization due to the solvent. Inclusion of the solvent effects reduces appreciably the energy difference between tautomers 1 and 3, but does not change their energetic ordering. Cl calculations indicate that the AMI method tends to underestimate the bond delocalization effects in the enone substructure of 1 so that correlation effects are important to describe the relative stability of 1 and 3

Semiempirical Study of Intramolecular Hydrogen Bond in Naphthazarin

It is found that MINDO/3, MNDO and AMI give a reasonable description of intramolecular hydrogen bonds in malonaldehyde, acetylacetone and naphtazarin tautomers as evidenced by comparison with the available ab initio results and reliable experimental f\u27indings. MINDO/3 and MNDO procedures of\u27Ier information at a qualitative level, whilst AMI yields semiquantitative resuIts after some empirical adjustements. The main result of this study is a strong indication that all studied systems possess asymmetric H-bond(s) at least in the gas phase. The most stable tautomer of naphthazarin has 1,4- -quinoid structure. It is found that single proton (two step) tunnelling is energetically much more favourable than concerted (simultaneous) transfer of both protons. Tunnelling barrier heights, of internal H-bonds are thoroughly discussed and interpreted in terms of the micro- and gross-energy partitioning method. Variation in structural parameters and changes in atomic charges are elaborated. It is established that keto- and hydroxy-oxygens have substantially different electron densities, which necessarily lead to appreciable differences in their ls ESCA shifts

A Semiempirical Study of Intramolecular Hydrogen Bonding in Some Naphthazarin Derivatives

Structural and energetic properties of some derivatives of naphthazarin are considered by the semiempirical AMI method. It is shown that these systems, possessing intramolecular hydrogen bonds, are qualitatively well described at the AMI level of theory. This conclusion is supported by a good qualitative agreement with available experimental data. The strength of the intramolecular hydrogen bond is briefly discussed. It appears that their energies are roughly additive

Theoretical Studies of the Solvent Effect on the Stability of the Ascorbic Acid Tautomers

AMI calculations on three conformers of each of the four tautomeric forms of ascorbic acid are reported. According to the results for isolated molecules, tautomer 3 should be lowest in energy, whereas experimentally only tautomer 1 is observed in the crystal as well as in solution. Solvent effects on the relative stability of the various structures are assessed by applying a combined quantum chemical and force field approach. It turned out to be very important to fully optimize both the water environment and the solute, which was achieved iteratively by means of a suitably modified AMI Hamiltonian taking into account the charge polarization due to the solvent. Inclusion of the solvent effects reduces appreciably the energy difference between tautomers 1 and 3, but does not change their energetic ordering. Cl calculations indicate that the AMI method tends to underestimate the bond delocalization effects in the enone substructure of 1 so that correlation effects are important to describe the relative stability of 1 and 3