Lithium parameters have been optimized for Stewart's standard PM3 method. The average deviation of the heats of formation calculated for 18 reference compounds is 6.2 kcal/mol from the experimental or high-level ab initio data; the average deviation with Li/MNDO is 18.9 kcal/mol. The average error in bond lengths is also reduced by a factor of two to three. Ionization potentials and dipole moments are reproduced with comparable accuracy than Li/MNDO. However, the mean deviation for the heats of formation of both methods increases when being applied to other systems, especially to small inorganic molecules

Optimization and application of lithium parameters for PM3

The origin of differential binding affinity and structural recognition between the inclusion complexes of cyclobis(paraquat-p-phenylene), 14+, and 1,4-substituted phenyl or 4,4‘-substituted biphenyl derivatives has been jointly determined by spectrometric techniques and ab initio and semiempirical molecular orbital methods. The unusual boxed geometry and tetracationic charge distribution in 14+ are key molecular features which produce strong intermolecular interactions with guest and solvent molecules. Solvation was addressed by including up to 12 acetonitrile molecules in the theoretical model, which realigned the predicted gas-phase supramolecular structures and energies into excellent agreement with experiment. The computed complexation enthalpies, ΔHbind, from the semiempirical molecular orbital PM3 method are on average within 1 kcal/mol of the experimental free energy binding data collected from absorption spectroscopy in acetonitrile. In addition, the computed geometric penetration and positioning ...

Unexpected Roles of Guest Polarizability and Maximum Hardness, and of Host Solvation in Supramolecular Inclusion Complexes: A Dual Theoretical and Experimental Study

Lithiation of 4,5-bis(methylsulfanyl)-TTF 9, 4,5-(ethylenedisulfanyl)-TTF 10, 4,5-dimethyl-TTF 11 and 4,5,5′-trimethyl-TTF 12 (TTF=tetrathiafulvalene) followed by reaction with methyl isothiocyanate affords the corresponding (N-methylthio-carbamoyl)-TTF derivatives 14–17, respectively, in 54–70% yields. These new TTF derivatives display a broad intramolecular charge-transfer band in their UV–VIS spectra arising from conjugation between the donor TTF ring and the acceptor N-methylthiocarbamoyl moiety. Steric hindrance between the adjacent N-methylthiocarbamoyl and methyl substituents in 17 causes a marked hyposchromic shift in this band (λmax 395 nm) compared to compounds 14–16 (lambda;max 435–467 nm). Consistent with the electron-withdrawing properties of the N-methylthiocarbamoyl substituent, its attachment to the TTF ring raises slightly the oxidation potential of the system. Charge transfer complexes of these donors and (N-methylthiocarbamoyl)-TTF 2 with 7,7,8,8-tetracyano-p-quinodimethane (TCNQ) and salts with bromide anions are reported, some of which have high room temperature conductivity values. The X-ray crystal structures are presented for 16, 17 and the salts 2·Br, 14·TCNQ and (17)2·20. The structure of 16 comprises orthogonal dimers (kappa packing) while in the structure of 17 individual molecules are orthogonal to each other. There is weak intermolecular hydrogen bonding in both 16 and 17. In the structure of 2·Br, the radical cations 2+· are almost planar and they form an infinite stair-like stack of dimers, with bromide anions situated between the stacks, and linked with the cation by a strong N–H‥Br bond. The structure of 14·TCNQ comprises mixed ‥DDAADD‥ stacks; the N-methylthiocarbamoyl group engages in an interstack N–H‥N bond with a TCNQ anion. Analysis of the bond lengths in the structure suggests that there is partial charge transfer from 14 to TCNQ. In the structure of (17)2·20 molecules form mixed ‥DDADDA‥ stacks and analysis of bond lengths suggests that there is only a small degree of charge transfer from donor to acceptor. The geometries of compounds 2, 14, 16, 17 were optimised using the PM3 semi-empirical method and the results compare favourably with the X-ray structural data.

(N-Methylthiocarbamoyl)tetrathiafulvalene derivatives and their radical cations: synthetic and X-ray structural studies

We report a comparative modified neglect of diatomic overlap (MNDO), Austin method one (AM1), and parametric method 3 (PM3) study of trans‐stilbene (tS) in its ground, excited (singlet and triplet), and ionic (positive and negative polarons and bipolarons) states We have also calculated the barrier for ring rotation about the backbone single bond Our results show that PM3 geometries are superior to MNDO and AM1, at least for tS PM3 predicts, in contrast with MNDO, AM1 and even ab initio 3‐21G, a coplanar structure for tS, in accordance with recent experimental data Singlet and triplet energies obtained from heats of formation are in surprisingly good agreement with experimental data

A parametric method 3 (PM3) study of trans‐stilbene

The synthesis of the new mono- and di-functionalized tetrathiafulvalene (TTF) derivatives 4, 5 and 12–15 is reported. Compounds 4 and 5 have been studied in detail: their single crystal X-ray structures are reported and theoretical calculations at both semiempirical and ab initio levels have been performed. The TTF moiety in the crystal of 4 is folded, whereas in the crystal of 5 it is essentially planar. In the crystal structure of 4 the molecules related via an inversion centre form close dimers which pack in a severely distorted kappa-arrangement. Molecules of 5 assemble into uniform stacks with TTF moieties overlapping the side chains of adjacent molecules. Calculated geometries are in good agreement with the X-ray data, and the distortion from planarity in the crystal of 4 can be explained by nonbonding interactions between the amide substituent and the TTF rings of adjacent molecules. Evidence for intramolecular charge-transfer from the TTF ring to the electron-withdrawing amide and ester substituents of 4 and 5, respectively, is provided by UV-VIS spectroscopy in solution, analysis of the bond lengths in the crystal structure, and by theoretical calculations.

New functionalized tetrathiafulvalenes: X-ray crystal structures and physico-chemical properties of TTF–C(O)NMe2 and TTF–C(O)–O–C4H9: a joint experimental and theoretical study

The ability of the MNDO, AM1, and PM3 semiempirical methods to reproduce pyramidalization at the nitrogen in 15 aromatic amines has been examined and compared to experimental and other theoretical results. AM1 consistently gives the best agreement. We have therefore reexamined the DNA intercalating compounds m-AMSA and o-AMSA using AM1.

Structural studies of aromatic amines and the DNA intercalating compounds m -AMSA and o -AMSA: comparison of MNDO, AMI, and PM3 to experimental and ab initio results

An in-depth computational study of the antitumor acridine m -AMSA (4'- (acridin-9-ylamino) - 3'-methoxymethanesulfonanilide) and its inactive analog o -AMSA (4'-(acridin-9-ylamino)-2'- methoxymethanesulfonanilide) was performed. Molecular mechanics calculations show that both compounds have similar minimum-energy structures in which the anilino ring bisects the plane of the acridine. The major difference found between the two compounds is in the orientation of the anilino ring methoxy group relative to the plane of the acridine ring. m -AMSA prefers a conformation in which the anilino ring methoxy is oriented away from the acridine while o -AMSA prefers the opposite configuration. Dynamics simulations of the conformational flexibility of the two compounds shows that both behave similarly. Gas-phase molecular dynamics studies of over 800 ps done in 100 ps runs show that there are no differences in the conformational states available to the two compounds. An examination of the electronic structures of the two drugs shows that the lowest unoccupied molecular orbital (LUMO) of m -AMSA is lower in energy than the LUMO of o -AMSA. This finding is interesting in light of the well-known dependency of the activity of these drugs on the electronic properties of the anilino ring. Further examination of the electronic structures of a variety of active and inactive compounds may prove useful in elucidating differences in electronic structure which might be correlated with drug activity.

Comparison of the 9-anilinoacridines o-AMSA (4'-(acridin-9-ylamino)-2'-methoxymethanesulfonanilide) and m-AMSA (4'-(acridin-9-ylamino)-3'- methoxymethanesulfonanilide). A molecular mechanics, dynamics and semiempirical molecular orbital study

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Structural studies of aromatic amines and the DNA intercalating compounds m -AMSA and o -AMSA: comparison of MNDO, AMI, and PM3 to experimental and ab initio results

Comparison of the 9-anilinoacridines o-AMSA (4'-(acridin-9-ylamino)-2'-methoxymethanesulfonanilide) and m-AMSA (4'-(acridin-9-ylamino)-3'- methoxymethanesulfonanilide). A molecular mechanics, dynamics and semiempirical molecular orbital study