A Critical Compilation of Substituent Constants

Exner, Otto

doi:10.1007/978-1-4615-8831-3_10

Cited by 175 publications

(55 citation statements)

References 231 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…(The substituent constants of I and R are 0.06 and À0.52 for (CH 3 ) 2 N, respectively, and are 0.01 and À0.11 for CH 3 , respectively.) 11 The negative charge densities of carbonyl and ether oxygens in II might be expected to be larger than those of corresponding oxygens in VIII because of the þR effect of the N atom. Actually, the charge densities on the carbonyl and ether oxygens obtained by natural population analysis 12 at the QCISD/6-31G(d) level are À0.73e and À0.68e for II and À0.69e and À0.64e for VIII, respectively.…”

Section: Ts Structuresmentioning

confidence: 99%

Effects of entropy on the gas‐phase pyrolysis of ethyl N,N‐dimethylcarbamate

Kim

Hsieh

et al. 2006

J Comput Chem

View full text Add to dashboard Cite

In this study, we examined the gas-phase pyrolysis of ethyl N,N-dimethylcarbamate theoretically at various theoretical levels. The reaction consists of a two-step mechanism, with N,N-dimethylcarbamic acid and ethylene as reaction intermediates. In the first step, the reaction proceeds via a six-membered cyclic transition state (TS), which is more favorable than that via a four-membered cyclic TS. Here, the contribution of entropy to the overall potential energy surface was found to play an important role in determining the rate-limiting step, which was found to be the second step when viewed in terms of the enthalpy of activation (DeltaH(not equal)), but the first step when entropy changes (-TDeltaS(not equal)) were considered. These results are consistent with experimental findings. Moreover, the experimental activation entropy can be reproduced by using the hindered rotor approximation, which converts some low vibration frequencies that correspond to internal rotational modes into hindered rotors.

show abstract

Section: Ts Structuresmentioning

confidence: 99%

Effects of entropy on the gas‐phase pyrolysis of ethyl N,N‐dimethylcarbamate

Kim

Hsieh

et al. 2006

J Comput Chem

View full text Add to dashboard Cite

show abstract

“…14 A parallel can be drawn here with the substituent effects in organic chemistry, where the main contributions are related to resonance or field/inductive effects. [15][16][17] The numerical characteristics (so-called substituent constants) 18,19 do not always work for systems being significantly different from those for which they were estimated. Numerous interpretations of a variety of chemical and physicochemical properties are present in modern handbooks on organic chemistry.…”

Section: Introductionmentioning

confidence: 99%

Metal Complexation and H-bonding Effects on Electronic Structure of Cytosine Studied in the Gas Phase

Stasyuk¹,

Szatyłowicz²,

Krygowski³

2014

Croat. Chem. Acta

View full text Add to dashboard Cite

Abstract.The influence of H-bonding and complexation with cations (probed by HF, F -, Li + , Na + and K + ) on structural and π-electron changes in the six most stable cytosine tautomers has been studied in the gas phase using the B3LYP/6-311++G(2d,2p) computational level. The presence of two exo-groups (amino/imino and carbonyl/hydroxyl) in cytosine tautomers significantly increases their sensitivity to structural changes due to intra-and intermolecular interactions. These interactions induce large changes in aromaticity of the rings and in the CX (X = N, O) bond lengths of exocyclic groups. Three types of H-bonds, considering their strength, could be distinguished: (i) charge-assisted X -···HF, X = N or O, as the strongest, (ii) neutral X···HF, where X is the nitrogen atom of the ring or imino group or the keto form oxygen atom and (iii) also neutral X···HF, where X being either amino N or alternatively hydroxylic O. Hydrogen bond energy decreases approximately twice in the above listed sequence of interactions. Structural consequences of H-bonding and metal complexation have been observed not only in the immediate region of the interaction but also in other parts of the molecule (the shape of the amino group, changes in CO and CN bond lengths). Complexation of the cytosine tautomers with cations leads to monotonic changes in aromaticity in line with an increase of their ionic radii.

show abstract

“…The effectiveness of a benzazole as an acti vating group may be estimated by comparison of its group Hammett coeffi cients relative to other common activating groups. In fact, it is fortunate that Hammett coefficients for some of these benzazoles has already been deter mined (19,20). These heterocycles have the following σ ρ values when appended at their 2-positions (on the carbon between the two electronegative heteroatoms): benzoxazole, 0.34; benzthiazole, 0.34; N-phenylbenzimidazole, 0.24.…”

Section: Resultsmentioning

confidence: 97%