Electron Delocalization in Isocyanates, Formamides, and Ureas: Importance of Orbital Interactions

Bharatam, Prasad V.; Moudgil, Rajnish; Kaur, Deepinder

doi:10.1021/jp027044+

Cited by 48 publications

(57 citation statements)

References 51 publications

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“…This imparts a partial double bond character to the CON bond. But recent experimental and theoretical studies [41][42][43][44] tell a different story. The electron delocalization in the amide system has been attributed to second-order orbital interactions namely, n O 3 * CON (delocalization from lone pairs on oxygen into the sigma antibonding orbital of the CON bond, i.e., negative hyperconjugation) and n N 3 * COO (delocalization from the lone pair on nitrogen to the pi-antibonding orbital of the carbonyl group).…”

Section: Rotation Barriers In Bma and Nmamentioning

confidence: 99%

Geometry, transition states, and vibrational spectra of boron isostere of N‐methylacetamide by ab initio calculations

Malde

Khedkar

Coutinho

et al. 2005

Int J of Quantum Chemistry

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ABSTRACT:The basic unit of the peptide, the COONH bond, is responsible for imparting specific secondary structural features to peptides. Many modifications, such as isosteric/bioisosteric replacement of this basic unit, have been made to alter the properties of peptides. Isosteric replacement of the nitrogen atom by boron (boron peptides) is another plausible way of changing peptide characteristics. Like Nmethylacetamide (NMA), acetylmethylborane, or BMA, is a good model for studying boron peptides. The potential energy surface (PES) of BMA has been studied by Hartree-Fock (HF), density functional theory (DFT), and post-HF methods. The PES of BMA is an inverted form of the NMA hypersurface. Two minima and two saddle points of index 1 have been identified on this PES and fully optimized at the HF, Becke's three-parameter exchange functional and the gradient-corrected functional of Lee, Yang, and Paar, second-order Møller-Plesset (full), and quadratic configuration interaction method with single and double substitutions followed by a perturbative treatment of triple substitutions (QCISD) levels of theory. The minima correspond to structures with "omega" values close to 90°and 270°, and the transition states have omega values around 0°and 180°. The energy barrier for rotation around the "omega bond" is found to be 4.3 kcal/mol at the QCISD/6-31G* level, which is much lower than the 16 -23 kcal/mol barrier in NMA. The unique shape of the PES of BMA and the low barrier to rotation can be well explained by second-order orbital interaction studies. Normal-mode analysis reveals a significant shift in the BOH stretching frequency of BMA from that in alkyl boranes and borane-phosphine complexes. The replacement of nitrogen by boron bestows on such peptides greater proteolytic stability, more flexibility around the omega angle, and a unique preference for positive angles, all of which are absent in peptides of natural origin.

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Section: Rotation Barriers In Bma and Nmamentioning

confidence: 99%

Geometry, transition states, and vibrational spectra of boron isostere of N‐methylacetamide by ab initio calculations

Malde

Khedkar

Coutinho

et al. 2005

Int J of Quantum Chemistry

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“…Moreover, the urea is of interest in experimental and theoretical investigations because of simplicity of its molecule [43][44][45][46], interesting highly symmetrical crystal structure [47,48] or physical properties [49,50]. In contrast, N-methylurea co-crystals are little recognized by X-ray methods and up to now only two co-crystal structures have been determined [51,52].…”

Section: Introductionmentioning

confidence: 99%

Co-crystal/salt crystal structure disorder of trichloroacetic acid–N-methylurea complex with double system of homo- and heteronuclear O–H···O/N–H···O hydrogen bonds: X-ray investigation, ab initio and DFT studies

Rybarczyk‐Pirek

2012

Struct Chem

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The X-ray diffraction studies revealed disorder of a trichloroacetic acid-N-methylurea complex crystal structure, connected with a proton transfer via O-HÁÁÁO hydrogen bond. The observed structure corresponds to a co-existence of ionic (salt) and neutral (co-crystal) forms of the complex in the solid state in ratio 3:1, respectively. The geometrical analysis based on ab initio and density functional theory methods combined with the experimental research indicated that two different N-methylurea molecular conformations, defined by CNCN torsion angle, correspond to the neutral and the ionic form of the complex, respectively. The conformational changes seem to be connected with stabilization of the ionic structure after a proton transfer, as according to theoretical calculations this form of the complex (the ionic one) was unstable in the gas phase. A particular attention was focused on a system of a double intermolecular hydrogen bonds, O-HÁÁÁO and N-HÁÁÁO which join molecules into the title complex. The analysis of these interactions performed in terms of their geometry, energetic and topological electron density properties let for their classification into strong and medium strength hydrogen bonds. It was also found that the antibonding hydrogen bonding donor orbital occupation corresponded to the stabilization energy resulting from charge transfer in hydrogen bonds. Hence, it is postulated as a possible indicator of interaction strength.

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“…A small amount of free catalyst 1a was observed, but no free DMAP was detected ( Figure S3). 11 Since the negative charge in 1a can delocalize between S and N atoms, 18 we suspect that the two species might be the S−Br and N−Br complexes (i.e., Figure 3, A and A′). Although zwitterion 1a and DMAP exist in equilibrium in the solution phase and could be active catalysts, as indicated in Table 1, NBS−1a complexes appear to be the dominant active species, since the NBS−DMAP complex was not observed in the NBS−1a 1 H NMR study ( Figure S4).…”

mentioning

confidence: 99%

Efficient Medium Ring Size Bromolactonization Using a Sulfur-Based Zwitterionic Organocatalyst

et al. 2012

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Electron Delocalization in Isocyanates, Formamides, and Ureas: Importance of Orbital Interactions

Cited by 48 publications

References 51 publications

Geometry, transition states, and vibrational spectra of boron isostere of N‐methylacetamide by ab initio calculations

Geometry, transition states, and vibrational spectra of boron isostere of N‐methylacetamide by ab initio calculations

Co-crystal/salt crystal structure disorder of trichloroacetic acid–N-methylurea complex with double system of homo- and heteronuclear O–H···O/N–H···O hydrogen bonds: X-ray investigation, ab initio and DFT studies

Efficient Medium Ring Size Bromolactonization Using a Sulfur-Based Zwitterionic Organocatalyst

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