Ab initio studies of the conformers and conformational distribution of gas‐phase N,N‐dimethylaminopropanol

Systematic and extensive conformational search has been performed to characterize the gas‐phase N,N‐dimethylaminopropanol structures. A total of 91 unique trail structures were generated by allowing for all the single‐bond rotamers. All the trial structures were initially optimized at the AM1 level, and the resulting structures were optimized at the B3LYP/6‐311G* level of theory and then subjected to further optimization at the B3LYP/6‐311++G**. A total of 36 conformers are found and their zero‐point vibrational enegies, rotational constants, and dipole moments are determined. Vertical ionization energies of 11 low‐lying conformers predicted with the electron propagator theory are in good agreement with the experimental data. The two most stable conformers display intramolecular H bonds (HBs): OH···N. These HBs influence on the molecular electronic structures is exhibited by natural bond orbital analyses. Combined with statistical mechanics principles, conformational distributions at various temperatures are computed and the temperature dependence of photoelectron spectra is interpreted. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2010     

Conformational search for zwitterionic leucine and hydrated conformers of both the canonical and zwitterionic leucine using the DFT-CPCM model

The stable conformations for zwitterionic leucine have been searched for in solution as well as in gas phase. A total of 54 trial structures were generated by considering possible combinations of single bond rotamers. It is observed that zwitterions are not stable in gas phase. In order to investigate the zwitterions of leucine in solution, the calculations for all trial structures of zwitterions were performed initially at the PM3 level and 14 the lowest energy structures were reoptimized at the B3LYP/6-311G(d) level using the CPCM model. Seven of these conformers of zwitterionic leucine were found to be stable in solution. The five most stable conformers were then reoptimized at the B3LYP/6-311++G(d, p) level. The energy ordering of the canonical leucine(neutral) conformers were also considered on the basis of single point energy calculations at the B3LYP/6-311++G(d, p) level using the CPCM model. The chemical hardness, chemical potential, vertical ionization energy and vertical electron affinity were calculated for a few of the most stable canonical leucine and its zwitterions in solution. The effects of explicit addition of water molecules (microsolvation) on the structure and the energy of both canonical and zwitterionic conformers of leucine were investigated. It is noted that in gas phase, the singly and doubly hydrated canonical (neutral) forms are more stable than their zwitterionic counterparts. The solvated zwitterions and canonical structures of leucine were further investigated using the discrete/SCRF model with zero, one and two water molecules. In solution, the continuum solvent model shows that the bare zwitterionic form is more stable than the bare canonical form by 1.6 kcal/mol. This energy separation is increased to 3.8 and 4.8 kcal/mol with inclusion of one and two water molecules, respectively. The optimized structural parameters for the most stable zwitterionic leucine with zero, one and two water molecules in solution were compared with those reported for l-leucine crystal, which shows a close agreement between the optimized geometrical parameters of the zwitterionic leucine with two water molecules in solution with the experimental geometrical parameters for l-leucine crystal. It is also observed that when the structures of zwitterions with one and two explicit water molecules are optimized in solution, the geometrical parameters and their relative energies are found to be appreciably modified. We have also calculated the vibrational spectra of the most stable solvated zwitterionic leucine as well as for the most stable structure of zwitterionic leucine with one and two water molecules in solution.     

Ionic liquid structure: the conformational isomerism in 1‐butyl‐3‐methyl‐imidazolium tetrafluoroborate ([bmim][BF4])

As a probe of local structure, the vibrational properties of the 1‐butyl‐3‐methylimidazolium tetrafluoroborate [bmim][BF₄] ionic liquid were studied by infrared (IR), Raman spectroscopy, and ab initio calculations. The coexistence of at least four [bmim]⁺ conformers (GG, GA, TA, and AA) at room temperature was established through unique spectral responses. The Raman modes characteristic of the two most stable [bmim]⁺ conformers, GA and AA, according to the ab initio calculations, increase in intensity with decreasing temperature. To assess the total spectral behavior of the ionic liquid both the contributions of different [bmim]⁺ conformers and the [bmim]⁺− [BF₄]⁻ interactions to the vibrational spectra are discussed. Copyright © 2008 John Wiley & Sons, Ltd.     

Mechanochromism,Twisted/Folded Structure Determination,and Derivatization of (N‐Phenylfluorenylidene)acridane

(N‐Phenylfluorenylidene)acridane (Ph‐FA) compounds with electron‐withdrawing and ‐donating substituents (H, MeO, Ph, NO₂, Br, F) at the para position of the phenyl group were successfully synthesized by Barton–Kellogg reactions of N‐aryl thioacridones and diazofluorene. By using the substituent on the nitrogen atom to alter the electronic properties, both the folded and twisted conformers of p‐NO₂‐C₆H₄‐FA could be crystallographically characterized, which enabled the charge transfer from the electron‐donating acridane moiety to the electron‐accepting fluorenylidene moiety to be understood. Ground‐state mechanochromism, thermochromism, vapochromism, and proton‐induced chromism were demonstrated between the folded and twisted conformations of the conformers. Protonation and chemical oxidation of Ph‐FA gave two stable acridinium compounds, namely, the fluorenylacridinium and acridinium radical cations. The present study will contribute to the development of functional dyes and organic semiconductors.     

First‐principles conformational analysis of the C36H36 spheriphane,a prototype hydrocarbon host cage

Comprehensive B3LYP/6‐311G** electronic structure calculations establish that, unlike closely related species such as cyclophanes, the C₃₆H₃₆ spheriphane (heptacyclo[13.13.2^1,15.2^8,22.1^3,27.1^6,10.1^13,17.1^20,24] hexatriaconta‐1,3(33),6,8,10(34),13,15,17(35),20,22,24(36),27‐dodecaene) possesses only seven energetically distinct conformers, out of which five exist in enantiomeric pairs and two are achiral. These local energy minima are interrelated through an intricate net of 20 reaction paths involving single inversions at the ? CH₂? CH₂? bridges. In particular, the T and C₃ conformers, which are predicted to coexist in comparable concentrations at ambient temperatures, are linked through three consecutive single‐bridge inversions that proceed through the overall barrier of only 5.3 (kcal/mol). This barrier is compatible with coalescence temperatures well below those employed in the recently measured ¹H and ¹³C NMR spectra, explaining the observed lack of line splittings. Both the T→C₃ pathway and the racemizations of the low‐energy conformers involve C₁ and C₂ intermediates that are expected to be present in detectable amounts. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 1279–1286, 2001     

Electron diffraction and quantum-chemical studies of the conformational properties of the 1,3-benzenedisulfochloride molecule

A combined electron diffraction (T = 394(5) K) and quantum-chemical (MP2/6-31G**) study has been performed to investigate the molecular structure of 1,3-benzenedisulfochloride (1,3-BDSC). The 1,3-BDSC molecule was found to exist as the trans (I) and cis (II) stable conformers where the planes containing S-Cl bonds are perpendicular to the plane of the benzene ring. The energy of conformer I is 0.13 kJ/mol lower than that of conformer II. The mutual effect of the sulfochloride groups was found to be absent, which is evident from the coincident bond lengths and angles in the two conformers. The main structural parameters of the conformers are r _h1(C-H)_av = 1.103(4) Å, r _h1(C-C)_av = 1.401(3) Å, r _h1(C-S) = 1.767(4) Å, r _h1(S=O) = 1.422(3) Å, r _h1(S-Cl) = 2.048(4) Å, ∠Cl-S-O = 106.6(2)°, ∠C-S-Cl = 100.4(5)°, ∠ O-S-O = 123.2(5)°.     

Impact of Intermolecular Non-Covalent Interactions in a CuI8PdII1 Discrete Assembly: Conformers’ Geometries and Stimuli-Sensitive Luminescence Properties

A new highly solid-state luminescent phase of a previously reported weakly luminescent Cu^I₈Pd^II₁ dicationic assembly is reported revealing the high geometrical versatility of this moiety that importantly alters its luminescent properties. This very minor new species B_c is based on a different conformer scaffold than the one encountered in the previously reported B_o form and, essentially differs from B_o by displaying shorter Cu^I-Cu^I intermetallic distances. DFT calculations allow concluding that the predominance in the solid-state of the weakly luminescent and less stable B_o phase is due to the extra stability induced by a larger number of intermolecular non-covalent π-CH interactions in its crystalline packing and not by the intrinsic stability of the Cu^I₈Pd^II₁ dicationic moiety. Calculations also revealed that a more stable conformation B_calc is expected in vacuum, which bears a different distribution of Cu^I-Cu^I intermetallic distances than the dications in B_o and B_c phases. Taking into account that the geometrical alterations are associated to drastic changes of luminescence properties, this confer to the Cu^I₈Pd^II₁ assembly high potentiality as stimuli-sensitive luminescent materials. Indeed, by applying mechanical or thermal stress to samples of B_o phase, new phases B_g and B_m , respectively, were obtained. Alterations of the solid-state photophysical properties of these new species compared to those recorded for B_o are reported together with a combined experimental and computed study of the structures/properties relationships observed in these phases.     

Understanding selenocysteine through conformational analysis,proton affinities,acidities and bond dissociation energies

Density functional methods have been employed to characterize the gas phase conformations of selenocysteine. The 33 stable conformers of selenocysteine have been located on the potential energy surface using density functional B3LYP/6‐31+G* method. The conformers are analyzed in terms of intramolecular hydrogen bonding interactions. The proton affinity, gas phase acidities, and bond dissociation energies have also been evaluated for different reactive sites of selenocysteine for the five lowest energy conformers at B3LYP/6‐311++G*//B3LYP/6‐31+G* level. Evaluation of these intrinsic properties reflects the antioxidant activity of selenium in selenocysteine. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008