Showing papers on "Molecular geometry published in 1993"

Self-assembly and photophysical properties of lanthanide dinuclear triple-helical complexes

Abstract: The dinucleating ligand bis[ 1- methyl-2-(6'- [ 1”-(3,5 imethoxybenzyl)benzimidazol-2~'-ylp]y rid-2'-y1)benzimidazol- 5-yllmethane (L) reacts with lanthanide perchlorates to give dinuclear 2:3 complexes [Lnz(L)#+ (Ln = La, Eu, Gd, Tb, and Lu). Detailed ES-MS, lH-NMR, luminescence, and spectrophotometric measurements in acetonitrile show that the cations [Ln2(L)#+ are produced by strict self-assembly and adopt a triple-helical structure in solution (pseudo-D3s ymmetry). The crystal structure of [EU~(L)~] C~O~)~((1C1H, E~UC2NC1)9s~H 177N39036_Cl6,a = 17.634- (3) A, b = 21.408(4) A, c = 29.437(7) A, a = 82.13(1)', /3 = 85.76(1)', y = 89.79(1)', triclinic, P1, Z = 2) shows a dinuclear pseudo-D3 triple- elical cation, [Eu2(L)#+, where the three bis(terdentate) ligands L are wrapped around the helical axis defined by the europium atoms. The Eu(II1) of each site is 9-coordinated by six nitrogen atoms of the benzimidazole units occupying the vertices and three nitrogen atoms of the pyridine units occupying the capping positions of a slightly distorted, tricapped trigonal prism. Luminescence studies of the crystalline complex [Eu~(L)~]- (ClO.n n = 2, solv = H20,6; n = 9, solv = H20,7) confirm the pseudo-D3 symmetry of the Eu(II1) sites in 11 and show that secondary interactions with water molecules in 6 and 7 destroy the trigonal symmetry. An efficient intramolecular energy transfer between the %A* excited state centered on L and the excited levels of Eu(II1) and Tb(II1) is observed (antenna effect) together with a dipole4ipolar Tb - Eu intramolecular energy transfer in the heterodinuclear-doped Eu-Tb compound. Stability constants and 1H NMR in acetonitrile show that the homodinuclear complexes [LI I ~ (L) ~ ]a~re+ l ess stable for the heavier lanthanides Tb and Lu. The origin of this effect is discussed together with the nonstatistical distribution of the different species observed when stoichiometric quantities of L (3 equiv) are mixed with Ln1(C104)3 (1 equiv) and L I I ~ ( C ~ O(1~ e)q~ui v) in solution (Ln1 # Ln2; Ln1 = La, Eu, Tb, and Lu; Ln2 = Tb and Lu).

Metal dependence of the nonplanar distortion of octaalkyltetraphenylporphyrins

Abstract: The biological activity of porphyrins and related tetrapyrroles in proteins may be modulated by nonplanar conformational distortions; consequently, two aspects of nonplanarity have been investigated in the highly nonplanar octaalkyltetraphenylporphyrins (OATPPs). In the first part, the effect of the central metal ion (M = Ni(II), Co(II), Cu(II), Zn(II), Co(III), Fe(III)) on the conformation of the OATPP macrocycle has been determined. Crystallographic studies reveal that the sterically encumbered, nonplanar porphyrin 2,3,7,8,12,13,17,18-octaethyl-5,10,15,20-tetraphenylporphyrin (OETPP) remains sufficiently flexible to show a small decrease in nonplanarity for large metal ions. This decrease in nonplanarity for the OETPP metal complexes is predicted by using a molecular mechanics force field derived from structural and vibrational data for planar metalloporphyrins. A detailed analysis of the crystal structures of the Co(II) and Cu(II) complexes of OETPP reveals that the metal-dependent changes in bond lengths and bond angles are qualitatively similar to the changes observed for the OEP complexes. As the metal size increases, both OEPs and OETPPs exhibit expansion of the meso bridges (increases in the C[sub [alpha]]-C[sub m] bond length and the C[sub [alpha]]-C[sub m]-C[sub [alpha]] bond angle) and a movement of the coordinating nitrogen atoms away from the metal atom (increases in the M-N bond length andmore » the C[sub [alpha]]-N-C[sub [alpha]] bond angle and a decrease in the N-C[sub [alpha]] bond length). In the second part, a combination of molecular mechanics and INDO/CI molecular orbital calculations successfully predicts the optical spectra of a series of highly substituted OATPPs with increasing nonplanar distortion. The successes of these calculations indicates the importance of including both the macrocycle conformation and the peripheral substituents in the INDO calculations. 62 refs., 10 figs., 6 tabs.« less

The structure of aniline by ab initio studies

Abstract: The structure of aniline has been studied by ab initio calculations. Complete geometry optimization of (1) the energy minimum structure and the transition states for (2) internal rotation and (3) inversion of the amino group were carried out at the SCF level using several different basis sets. For these three stationary geometries vibrational frequencies were calculated at the SCF/6-31G∗∗ level. The effect of electron correlation was estimated by single point MP4(SDQ) calculations using the 6-311G∗∗ basis set. To satisfactorily describe the conformation and orientation of the amino group a fully polarized (6-31G∗∗) basis set is required. It is predicted that the aniline molecule has a pyramidal amino group with an angle between the C-N bond and the NH2 plane of 42.3°. The angle between the C-N bond and the plane of the benzene ring is 2.0°. The barriers to inversion and internal rotation of the amino group are estimated to be 1.7 and 3,7 kcal mol− respectively.

Simulated annealing study of neutral and charged clusters: Aln and Gan

Abstract: Density functional calculations with simulated annealing have been performed for clusters of aluminum Aln and gallium Gan up to n=10. There are many local minima in the energy surfaces, with a rich variety of structures and spin multiplicities. With increasing cluster size we find transitions from planar to nonplanar structures at n=5, and to states with minimum spin degeneracy at n=6. Isomers (n≥5) with buckled planar structures reminiscent of the layers in crystalline α‐gallium are generally less stable than ‘‘three‐dimensional’’ isomers. All structures show regular patterns of bond and dihedral angles. Systematic differences between Al and Ga clusters—bonds in the latter are shorter and bond angles closer to 90°—can be understood in terms of atomic properties. Trends in binding and ionization energies are compared with experiment and with the predictions of other calculations.

Ab initio molecular orbital cluster studies of the zeolite ZSM-5. 1. Proton affinities

Abstract: In this study we carried out ab initio molecular orbital calculations using the STO-3G, 3-21G, and 6-31G basis sets on aluminosilicate clusters representing ZSM-5 containing up to 46 tetrahedrally coordinated tetravalent atoms and including (SiO)[sub n] rings. The proton affinity and OH stretching frequency of the central bridging oxygen site (O(24)site) are examined as a function of cluster size and shape. The proton affinity is found to depend significantly (5-15 kcal/mol) on the shape of the cluster. Calculation of electrostatic potential in the region of the central bridging oxygen site for the clusters indicates that this dependence is largely due to electrostatic effects. The effects of geometry relaxation are studied with a constant-volume relaxation method in which a part of the cluster is optimized with the remaining being fixed in its crystal structure. Results indicate that relaxation increases the proton affinity by about 10-15 kcal/mol over the value from the rigid experimental structure. The final calculated proton affinity for the O(24) site in ZSM-5, including correction for zero-point effects, O:Si ratio, and higher level theory, is consistent with experiment. The OH stretching vibrational frequency is in good agreement with the experimental value. 53 refs., 13 figs., 6 tabs.

Molecular orbital study of acetic acid aggregation. 1. Monomers and dimers

Abstract: Ab initio and semiempirical (AM1, PM3, and SAM1) molecular orbital studies on monomeric and dimeric acetic acid are reported. The highest-level ab initio calculations (MP2/6-31 1++G(d,p)) on the monomeric forms of acetic acid predicted the cis form to be preferred over the trans by 6.1 kcal/mol. Four different dimers were studied. The enthalpy of stabilization at 298 K is 11.8 kcal/mol at the MP2/6-31 G(d) level after correction for BSSE and ZPVE for the cyclic dimer containing two O-H...O hydrogen bonds, which is 2.43.1 kcal/mol more stable than twice the open dimer containing only one O-H...O interaction. We attribute this difference to cooperativity in the cyclic structure. Vibrational analyses at the MP2/6-31 G(d) level confirm this assessment

Tautomeric equilibrium and hydrogen shifts of tetrazole in the gas phase and in solution

Abstract: High-level ab initio molecular orbital calculations, using basis sets up to 6-311+G(2d,2p) with electron correlation incorporated at the quadratic configuration interaction [QCISD(T)] level, have been used to study the tautomeric equilibrium and hydrogen shifts of tetrazole in the gas phase and in solution. The solvent effects were investigated by self-consistent reaction field (SCRF) theory. Consistent with experimental observations, the 1H-tetrazole (1)/2H-tetrazole (2) tautomeric equilibrium is calculated to be strongly influenced by the surrounding medium. 2H-Tetrazole is the energetically preferred tautomer in the gas phase. In a nonpolar solution, both the 1H and 2H forms are predicted to exist in comparable amounts. However, in a medium of high dielectric constant the more polar 1H tautomer is the dominant species. The calculated free energy changes for tautomerization of IH-tetrazole in the gas phase and in nonpolar (epsilon = 2) and polar (epsilon = 40) media are -7, 1, and 12 kJ mol-1, respectively. The molecular geometry, charge distribution, and vibrational frequencies of the polar 1H tautomer are found to be altered significantly in the presence of a solvent reaction field. Isomerization of 1 to 2, via a [1,2] hydrogen shift, requires an energy barrier of 207 kJ mol-1 in the gas phase. 5H-Tetrazole (3) is predicted to lie 82 kJ mol-1 above 1, due to its nonaromatic character. However, rearrangement of 3 to 1, via a [1,5] hydrogen shift, is inhibited by an activation barrier of 150 kJ mol-1. Conversely, the energy barrier for the rearrangement of 1 to 3 is 232 kJ mol-1, slightly larger than that required for the isomerization of 1 to 2. These results suggest that 3 is a good candidate for experimental observation. Inclusion of electron correlation leads to a drastic change in the molecular geometry of 3. At the MP2 level, an acyclic structure is predicted, while at the MP3 and QCISD levels the expected cyclic structure is found, The calculated molecular geometry of 1H-tetrazole at the MP2 level is found to differ significantly from the available solid-state structural data.

Fluxionality and low‐lying transition structures of the water trimer

Abstract: The minimum energy structure of the cyclic water trimer, its stationary points, and rearrangement processes at energies <1 kcal/mol above the global minimum are examined by ab initio molecular orbital theory. Structures corresponding to stationary points are fully optimized at the Hartree–Fock and second‐order Mo/ller–Plesset levels, using the 6‐311++G(d,p) basis; each stationary point is characterized by harmonic vibrational analyses. The lowest energy conformation has two free O–H bonds on one and the third O–H bond on the other side of an approximately equilateral hydrogen‐bonded O...O...O (O3) triangle. The lowest energy rearrangement pathway corresponds to the flipping of one of the two free O–H bonds which are on the same side of the plane across this plane via a transition structure with this O–H bond almost within the O3 plane. Six distinguishable, but isometric transition structures of this type connect six isometric minimum energy structures along a cyclic vibrational‐tunneling path; neighboring...

Molecular orbital studies of C-H...O H-bonded complexes

Abstract: Fully optimized ab initio and semiempirical molecular orbital calculations are reported on complexes containing C-H...O interactions that are prototypes of interactions commonly found in crystals. The ab initio calculations were performed both at the Hartree-Fock (HF) and second-order MOller-Plesset (MP2) levels using the 6-31G(d,p) and D95++(d,p) basis sets. The semiempirical calculations used the AM1, PM3, and SAM1 methods. The complexes considered are those of acetylene or hydrogen cyanide with water, formaldehyde, and ozone. The interaction energies, geometries, and vibrations are presented with corrections for zero-point vibration energy (ZPVE), basis set superposition error (BSSE), and enthalpy at 298 K, where appropriate

Valence Bond Concepts Applied to the Molecular Mechanics Description of Molecular Shapes. 2. Applications to Hypervalent Molecules of the P-Block

Abstract: A fascinating aspect of inorganic chemistry is the occurrence of complicated and varied molecular shapes. However, these same features lead to difficulties in developing molecular mechanics (MM) methods that are suitable for inorganic molecules. In this paper we demonstrate that simple valence bond concepts can guide the construction of a new MM force field for hypervalent molecules of the p-block of the periodic table. The primary difficulty in applying valence bond concepts to the MM description of hypervalent molecular shapes is the occurrence of intrinsically delocalized bonding arrangements, such as the three-center four-electron bond of XeF2. The inclusion of resonating configurations into the MM method provides a mechanism for surmounting the difficulties presented by hypervalent molecules. By making the contributions of the individual configurations to the total potential energy function dependent on the molecular geometry, we find that both equilibrium geometries and fluxional pathways of hyperva...

Water adsorption on zeolites : ab-initio interpretation of IR data

Abstract: Ab-initio 6-31G * OH frequency calculations show the water molecule complex with the bridging OH group of zeolites to be hydrogen-bonded. The experimental 3390- and 3695-cm -1 frequencies of the complex are interpreted to belong to the water OH group interacting with the surface by two hydrogen bonds and to the other slightly perturbed water OH group, respectively. The broad ∼2900-, ∼2450-, and ∼1700-cm -1 bands are explained on the basis of a theory of OH band profiles of hydrogen-bonded complexes to be the so-called (A, B, C) trio, caused by resonant interactions between the ν(OH)±ν(OH...O) combination modes and the δ(OH) and γ(OH) overtones of the perturbed bridging OH group

A density functional study on the strength of the metal bonds in Co2(CO)8 and Mn2(CO)10 and the metal-hydrogen and metal-carbon bonds in R-Mn(CO)5 and R-Co(CO)4

Abstract: Density functional studies have been carried out on the binuclear complexes (CO) 4 Co-Co(CO) 4 and (CO) 5 Mn-Mn(CO) 5 as well as the derivatives R-Mn(CO) 5 [R=H>CH 3 >CH 2 F>CHF 2 >CF 3 >C(O)CH 3 , and C(O)H] and R-Co(CO) 4 [R=H, CH 3 ]. The density functional calculations were based on the local density approximation (LDA) with nonlocal corrections for exchange and correlation included self-consistently (LDA/NL). The geometries optimized by LDA/NL agrees well with experimental structures. The deviations are 0.015 A for M-M and M-ligand distances and 2 o for ligand-metal-ligand bond angles

Ab initio molecular orbital calculations of the protonation reaction of propylene and isobutene by acidic OH groups of isomorphously substituted zeolites

Abstract: Ab initio molecular-orbital calculations using the 3-21G basis set are performed to study the protonation reaction of propylene and isobutene by zeolite bridged hydroxyls ZOH, which are simulated by a cluster model which consists of two Si tetrahedra and one Al tetrahedron. We have extended this study to include clusters with different compositions by introducing B and Ga in T III positions. The calculations show that in all reactions the adsorption of the olefin molecule on the acidic OH group takes place, leading to a stable π-complex with a structure similar to those of the isolated olefin and clusters constituents. The π-complex is transformed into a zeolite-alkoxide of covalent characteristics

Heats of formation of silicon hydride oxide (SiHnO and SiHnO2) calculated by ab initio molecular orbital methods at the G-2 level of theory

Abstract: To help facilitate the study of the energetics and the mechanism of silane combustion and the oxidation of related silicon species, H m Si-OH n , H m O-SiH n -OH p , and H m Si-O-OH n have been examined by ab initio molecular orbital methods. Geometries have been optimized at the MP2/6-31G(d) level of theory, and vibrational frequencies have been computed at HF/6-31G(d). Heats of formation have been calculated at the G-2 level of theory (estimated mean absolute error of ±2 kcal/mol or less)

Unusual attractive interactions between selenium and oxygen in selenoiminoquinones

Abstract: The reaction of benzeneseleninic anhydride and hexamethyldisilazane gives a reactive intermediate namely oligomeric (RSeN) 4 which oxidizes a phenol to the selenoiminoquinone 1. Both spectroscopic and crystallographic studies showed that the oxygen atom of the carbonyl group in the (phenylselenoimino)quinone 2 is involved in an attractive interaction with the selenium atom. The carbon atom of the phenyl ring is also found to be in the same plane as the O-C-C-N-Se plane. Theoretical studies of these systems clearly show a strong dependence of the Se-O interactions on the substituent on the selenium atom

The water–methanol complexes. I. A matrix isolation study and an ab initio calculation on the 1‐1 species

Abstract: The infrared spectrum of the CH3OH:H2O complex isolated in a nitrogen matrix is here reported. The complex is identified through observation of all three internal modes of the water moiety and 10 out of 12 vibrations of the methanol subunit. Several conformers of the CH3OH...OH2 are evidenced through ir photochemical and thermal conversion process, but none corresponds to the reverse CH3HO...HOH structure, with water playing the role of the proton donor. Ab initio calculations on both structures have been performed to obtain their equilibrium geometries and vibrational spectra. They allow to account for the ir spectral changes (frequencies as well as intensities) of the two submolecules interacting through hydrogen bonding within the complex.

Ab initio calculations on the geometry and OH vibrational frequency shift of cyclic water trimer

Abstract: The equilibrium geometrical parameters R OO θ and χ of cyclic water trimer have been determined at the counterpoise corrected SCF+MP2 level in the ESPB basis within pseudo-C 3v symmetry. The final structure has short ( R OO =2.85 A) and strongly bent (θ=20°) hydrogen bonds. The non-bonded OH bonds are directed by χ=48° out of the OOO plane. The interaction energy (Δ E ) is −14.7 kcal mol −1 and the corresponding D 0 =10.2 kcal mol −1 . The likely error bars on these results are discussed. The second order polarization interactions in the trimer are markedly non-additive. The total non-additivity contributes −2.0 kcal mol −1 to the final Δ E , and it is responsible for a shortening of R OO by 0.07 A. Its largest effect (about −70 cm −1 ) is in the H-bonded OH vibrational frequency shift Δν OH , which at the equilibrium geometry is calculated to be −230 cm −1 . The shift is markedly sensitive to the angle χ, and vibrational averaging along this coordinate is expected to reduce Δν. The results therefore support Nelander's reassignment of the IR and Raman gas phase OH spectra, which implies Δν OH ≈ −175 cm −1 .

Calculation of NMR chemical shifts: the third dimension of quantum chemistry

Abstract: NMR chemical shift calculations provide the basis for an intensive collaboration between quantum chemists and experimentalists. Calculated shift data can be used to describe the magnetic properties of a molecule, to identify unknown compounds by comparison of experimental and theoretical shift values, to determine equilibrium geometries, to investigate conformational changes, to elucidate the mechanism of molecular rearrangements, to determine solvent effects on NMR data, to identify complexation or coordination of soluted molecules by solvent molecules, to detect electronic structure changes caused by the medium, and to describe chemical bonding. This is demonstrated by three examples, namely the determination of the equilibrium structure of the homotropylium cation, the description of BH3NH3 in solution or condensed phases, and the investigation of stannyl cation complexes in solution. IGLO calculations of 13C, 11B, 15N, and 119Sn chemical shifts with DZ+P or TZ+P basis sets lead to the following results: (1) The homotropylium cation possesses an equilibrium 1,7 distance of 2 A that is indicative of strong through-space interactions and, as a consequence, homoaromatic character. (2) In solution, the charge transfer from NH3 to BH3 is increased, which leads to a decrease of the BN bond length, an increase of the dipole moment, and a shielding of both the B and the N nucleus. The experimental δ(11B) and δ(15N) values can be reproduced when the geometry effect and the direct solvent effect are included in the shift calculations. (3) Stannyl cations form strongly-bounded coordination complexes with solvent molecules (binding energy: ≥ 50 kcal/mol) that make the cation properties, in particular δ(119Sn) values, similar to those of covalently-bounded stannyl compounds. An experimental detection of stannyl cations in solution by NMR spectroscopy should only be possible by extensive solvent variations.

Molecular structure of N,N-dimethylformamide from gas-phase electron diffraction

Abstract: The molecular structure of gaseous N,N-dimethylformamide has been determined by electron diffraction, yielding the following bond lengths (r g ) and bond angles (r α ) with estimated total errors: (C-H) mean , 1.112±0.003 A; C=O, 1.224±0.003 A; (C(H 3 )-N) mean , 1.453±0.004 A; C(O)-N, 1.391±0.007 A; N-C=O, 123.5±0.6 o ; N-C(O)-H, 117.0±2.8 o ; (N-C(H 3 )-H) mean , 110.1±0.3 o ;C(H 3 )-N-C(H 3 ), 113.9±0.5 o ; (C(O)-N-C(H 3 )) mean , 121.6±0,3 o

Complex formation between water and formamide

Abstract: The infrared spectrum of the water–formamide complex in argon matrices has been recorded from 10 to 4000 cm−1. The interaction energy of the complex forming molecules has been calculated from a theoretical potential. One global and three different local minima have been found for this potential. Intermolecular vibration frequencies have been calculated for each minimum. The results are compared with the experimentally observed far infrared spectrum. In agreement with microwave measurements and ab initio calculations, the global minimum of the complex is found, both from calculations and experiment, to have a cyclic structure with water forming a hydrogen bond to the amide oxygen and receiving a hydrogen bond from an amide hydrogen. In addition to the cyclic complex, we observe one of the local minimum structures of the complex, where water accepts a hydrogen bond from the amide NH on the CH side of the amide.

Chemical shifts in proteins : an ab initio study of carbon-13 nuclear magnetic resonance chemical shielding in glycine, alanine, and valine residues

Abstract: Using gauge-including atomic orbital self-consistent field ab initio quantum chemical methods, we have computed the effects of bond lengths, bond angles, and torsion angles on the carbon- 13 chemical shielding of C α (and C β ) sites in model fragments for glycine, alanine, and valine residues in proteins. Predicted chemical shieldings are highly sensitive to bond length variations, and we show that it is essential to relax or energy minimize protein structures (to remove large errors associated with bond length uncertainties) in order to sucessfully predict experimental 13 C NMR spectra

Intramolecular N–H ⋯ O hydrogen bonding assisted by resonance. Part 2. Intercorrelation between structural and spectroscopic parameters for five 1,3-diketone arylhydrazones derived from dibenzoylmethane

Abstract: The crystal structures of five propane-1,2,3-trione arylhydrazones are reported. All molecules are chelated to form a six-membered π-conjugated ring via strong intramolecular N–H ⋯ O hydrogen bonding. The N ⋯ O hydrogen bond distances are correlated with the resonance entity within the ring and with spectroscopic data such as ν(NH)IR frequencies δ(NH)1H NMR chemical shifts and λmax UV absorption bands of charge transfer from hydrazone to carbonyl group. The structural and spectroscopic variations of the hydrogen bond parameters are modulated by the electronic properties of substituents on the aryl group in the sense that electron donating groups produce the strongest hydrogen bonds. The intercorrelation between N ⋯ O hydrogen bond strength and π delocalization in all the structures of this class retrieved from the Cambridge Structural Database shows that the interplay between π resonance and hydrogen bond magnitude, which we have called Resonance Assisted Hydrogen Bonding (RAHB), is a general phenomenon in the whole class of 1,3-diketone arylhydrazones.

Precise determination of the molecular geometry in fullerene C60 powder: A study of the structure factor by neutron scattering in a large momentum-transfer range.

Abstract: The molecular structure of fullerene C 60 has been determined with high precision using neutron scattering over a large range of momentum-transfer values. In the high-temperature plastic phase, at 295 K, the description of the complete structure factor in the 0-20-A -1 range, including Bragg and diffuse intensities, confirms the free reorientation of the C 60 spherical molecules. This analysis gives the carbon-carbon bond length within the five-member ring (single bond) equal to 1.4527(7) A, and that connecting five-member rings (double bond) equal to 1.3909(10) A

Density functional study of cluster models of zeolites. 1. Structure and acidity of hydroxyl groups in disiloxane analogs

Abstract: In this work we calculate the structures and acidities of a series of clusters that mimic terminal and bridging hydroxyl groups in zeolites and molecular sieves using the local density functional (LDF) program DMol. The clusters include silanol, H[sub 3]Si-OH, and its anion, the simplest model of terminal hydroxyl groups at zeolite surfaces and defect sites. We also consider disiloxane, H[sub 3]Si-O-SiH[sub 3], and a series of structural analogs of disiloxane, H[sub 3]T-O(H)-TH[sub 3] (T = tetrahedrally coordinated atom) in which Si is substituted by Al, B, P, Ga, or Ge. The accuracy of these LDF calculations is determined in part by the size of the numerical basis set and quadrature grid. We find that the structures of the clusters are insensitive to increases in quadrature grid size beyond [approx equal] 3000 points/atom. However, the optimized values of the internal coordinates, particularly the T-O-T bond angle, are much more sensitive to basis set size. The largest basis set used in these calculations (DNP+) shows convergence in key internal coordinates to [approx equal] 0.1[degree]-5.0[degrees] for T-O(H)-T bond angles and better than 0.01 A for T-O bond lengths. We also gauge the accuracy of the LDF results against extended basis set Hartree-Fockmore » MP2 results (MP2/DZ+2d) and experimental data. 60 refs., 5 figs., 21 tabs.« less

Theoretical study of N-methylacetamide in vacuum and aqueous solution : implications for the peptide bond isomerization

Abstract: A theoretical study on the changes in energy and charge distribution upon rotation of the amide bond of N-methylacetamide is presented. Both gas-phase and aqueous environments have been simulated in semiempirical (AM1) and ab initio (STO-3G, 6-31G and 6-31G * ) calculations with inclusion of electron correlation effects at the Moller-Plesset level. Relevant changes in the charge distribution are found during rotation of the peptide bond. The inclusion of water largely affects not only the energetics and the pathway of the amide twisting but also the charge distribution of the different conformers. Results are in good agreement with available experimental data and with results derived from high-level theoretical calculations. The chemical and biochemical importance of the results is discussed