Ab initio studies of structural features not easily amenable to experiment

Williams, John O.; Scarsdale, J.N.; Schäfer, Lothar; Geise, H. J.

doi:10.1016/0166-1280(81)85109-3

Cited by 87 publications

(9 citation statements)

References 46 publications

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“…No experimental structure is available for this molecule, but the present results accord with those calculated using a 4-21G basis. 39 The values obtained with the 6-31G** basis currently represent the best theoretical prediction for the geometry of this conformer of methanediol. Geometries optimized using the same three bases for transition structure FWU are given in Table I and are illustrated by PLUT040 drawings in Figure 1.…”

Section: Structural Featuresmentioning

confidence: 99%

Determination and characterization of a transition structure for water–formaldehyde addition

1983

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Quantum mechanical calculations of the geometric, energetic, electronic, and vibrational features of a transition structure for gas-phase water-formaldehyde addition (FWlt) are described, and a new transition-structure search algorithm is presented. Basis-set-dependent effects are assessed by comparisons of computed properties obtained from self-consistent field (SCF) molecular orbital (MO) calculations with STo-3G, 4-31G, and 6-31G** basis sets in the absence of electron correlation. The results obtained suggest that STO-3G-level calculations may be sufficiently reliable for the prediction of the transition structure of FWlt and for the transition structures of related carbonyl addition reactions. Moreover, the calculated activation energy for formation of FWlt from water and formaldehyde (-44 kcal mol-') is very similar in all three basis sets. However, the energy of formaldehyde hydration predicted by STO-3G (--45 kcal mol-l) is about three times larger than that predicted by the other two basis sets, with the activation energy for dihydroxymethane dehydration also being too large in STO-3G. Calculated force constants in all three basis sets are generally too large, leading to vibrational frequencies that are also too large. However, uniformly scaled force constants (in internal coordinates) give much better agreement with experimental frequencies, scaled 4-31G force constants being slightly superior to scaled STO-3G force constants.

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Section: Structural Featuresmentioning

confidence: 99%

Determination and characterization of a transition structure for water–formaldehyde addition

1983

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“…Un ensemble de calculs ab initio et semi-empiriques sur des mol6cules modules pouvant refl&er la g6om6trie de la r6gion anom6rique a apport6 une justification 6nerg6tique (Jeffrey, Pople & Radom, 1972, 1974Jeffrey, Pople, Binkley & Vishveshwara, 1978;Tvaroska & Kozar, 1980;Williams, Scarsdale, Schafer & Geise, 1981 ).…”

Section: Introductionunclassified

Etude structurale et densité de déformation électronique X–N à 75 K dans la région anomère du β-DL-arabinose

Longchambon¹,

Gillier‐Pandraud²,

Wiest³

et al. 1985

Acta Crystallogr Sect B

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The crystal structure of /3-DL-arabinose has been redetermined at 75 K, both by X-ray and by neutron diffraction. The cell dimensions and atom coordinates are in good agreement in both determinations, a = 5.8732 (8) [5.873(2)], b=7.7704(10) [7.762(2)], c= 13.2478(21) [13.243(9)] A,/3 = 99.96(I) [100.03(2)] °, V = 595.472 [594.55] .~3, Dr, = 1"674 [1 "677] Mg m -3, A = 0"7093 [0.8400] A (neutron diffraction values in square brackets). Electron deformation density maps around O atoms and the anomeric C atom have been computed by X-N methods. Experimental data concerning C-O bonds and O lone pairs are compared to theoretical ab initio studies on anomeric and exoanomeric effects. The most important points are: (i) lone pairs of O atoms 0(5) and O(1) seem to be quite identical except d2 of O(1) which interacts with the C(1)-O(5) bond; (ii) electron deformation density of anomeric C(5)-O(5) and O(1)-H bonds seems to be smaller than means in C-O and O-H bonds. These phenomena are consistent with predictions based on theoretical studies of anomeric and exoanomeric effects.

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“…For example, the half-life for azapeptide p-nitrophenolates at pH =7 is of the order of a few (8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20) minutes [60]. Moreover, the phenolic leaving groups would be of little use for inhibiting proteases, such as viral proteases that are specific for certain amino acids in the S binding subsites.…”

Section: Protease Inhibitors and Active Site Titrantsmentioning

confidence: 99%

“…The most interesting structural variation is associated with the hydrazine moiety. The conformations of hydrazine and its 1,2-diformyl derivative, which resemble the hydrazine moiety in azapeptides, were investigated by quantum chemical studies [19][20][21][22][23]. Calculations show that the global minimum energy structure of 1,2-diformyl hydrazine and its N-substituted derivatives is the nonplanar structure in which the nitrogen lone pairs are perpendicular to one another.…”

Section: Conformational Propertiesmentioning

confidence: 99%

Azapeptides as Pharmacological Agents

Zega¹

2005

Current Medicinal Chemistry

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Azapeptides, formed by replacing the C α of amino acid residues by nitrogen, are promising peptidomimetic compounds. Azaamino acids impart a unique conformational property to peptide structures because of the loss of chirality and reduction of flexibility of the parent linear peptide. The peculiar conformational properties make azaamino acids an attractive tool for drug design involving specific secondary structures in peptides and proteins. Additionally, since azapeptides are less susceptible to enzymatic breakdown by proteases, they may possibly lead to orally active drugs with longer duration of action. One of the advantages of azapeptides is their unproblematic synthesis allowing retention of the amino acid side chain. Azapeptides have been developed by several groups for the design of hormone analogues, protease inhibitors and active site titrants.

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Ab initio studies of structural features not easily amenable to experiment

Cited by 87 publications

References 46 publications

Determination and characterization of a transition structure for water–formaldehyde addition

Determination and characterization of a transition structure for water–formaldehyde addition

Etude structurale et densité de déformation électronique X–N à 75 K dans la région anomère du β-DL-arabinose

Azapeptides as Pharmacological Agents

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