TY  - JOUR
A1  - Kolocouris, Antonios
A1  - Koch, Andreas
A1  - Kleinpeter, Erich
A1  - Stylianakis, Ioannis
T1  - 2-Substituted and 2,2-disubstituted adamantane derivatives as models for studying substituent chemical shifts and C-H-ax center dot center dot center dot Y-ax cyclohexane contacts-results from experimental and theoretical NMR spectroscopic chemical shifts and DFT structures
JF  - Tetrahedron
N2  - The complete H-1 and C-13 NMR chemical shifts assignment for various 2-substituted and 2,2-disubstituted adamantane derivatives 1-38 in CDCl3 solution was realized on the basis of NMR experiments combined with chemical structure information and DFT-GIAO (B3LYP/6-31+G(d,p)-GIAO) calculations of chemical shifts in solution. Substituent-induced C-13 NMR chemical shifts (SCS) are discussed. C-H-ax center dot center dot center dot Y-ax contacts are a textbook prototype of steric hindrance in organic chemistry. The nature of these contacts will be further investigated in this work on basis of new adamantane derivatives, which are substituted at C-2 to provide models for 1,4-C-H-ax center dot center dot center dot Y-ax and 1,5-C-H-ax center dot center dot center dot Y-ax contacts. The B3LYP/6-31+G(d,p) calculations predicted the presence of NBO hyperconjugative attractive interactions between C-H-ax and Y-ax groups along C-H-ax center dot center dot center dot Y-ax contacts. The H-1 NMR signal separation, Delta delta(gamma-CH2), reflects the strength of the H-bonded C-H-ax center dot center dot center dot Y-ax contact. (C) 2015 Elsevier Ltd. All rights reserved.
KW  - 2-Substituted adamantane derivatives
KW  - 2,2-Disubstituted adamantane derivatives
KW  - H-1 NMR
KW  - C-13 NMR
KW  - B3LYP/6-31+G(d,p) calculations
KW  - GIAO calculations
KW  - Substituent chemical shifts
Y1  - 2015
U6  - https://doi.org/10.1016/j.tet.2015.01.044
SN  - 0040-4020
VL  - 71
IS  - 16
SP  - 2463
EP  - 2481
PB  - Elsevier
CY  - Oxford
ER  - 
TY  - JOUR
A1  - Kolocouris, Antonios
A1  - Zervos, Nikolaos
A1  - De Proft, Frank
A1  - Koch, Andreas
T1  - Improper Hydrogen Bonded Cyclohexane C-H-ax center dot center dot center dot Y-ax Contacts theoretical predictions and experimental Evidence from H-1 NMR Spectroscopy of Suitable Axial Cyclohexane Models
JF  - The journal of organic chemistry
N2  - C-H-ax center dot center dot center dot Y-ax are a textbook prototype of steric hindrance in organic chemistry. The nature of these contacts is investigated in this work. MP2/6-31+G(d,p) calculations predicted the presence of improper hydrogen bonded C-H-ax center dot center dot center dot Y-ax of different strength in substituted cyclohexane rings. To support the theoretical predictions with experimental evidence, several synthetic 2-substituted adamantane analogues (1-24) with suitable improper H-bonded C-H-ax center dot center dot center dot Y-ax contacts of different strength were used as models of a substituted cyclohexane ring. The H-1 NMR signal separation, Delta delta(gamma-CH2), within the cyclohexane ring gamma-CH(2)s is raised when the MP2/6-31+G(d,p) calculated parameters, reflecting the strength of the H-bonded C-H-ax center dot center dot center dot Y-ax contact, are increased. In molecules with enhanced improper H-bonded contacts C-H-ax center dot center dot center dot Y-ax, like those having sterically crowded contacts (Y-ax = t-Bu) or contacts including considerable electrostatic attractions (Y-ax = O-C or O=C) the calculated DFT steric energies of the gamma-axial hydrogens are considerably reduced reflecting their electron cloud compression. The results suggest that the proton H-ax electron cloud compression, caused by the C-H-ax center dot center dot center dot Y-ax contacts, and the resulting increase in Delta delta(gamma-CH2) value can be effected not just from van der Waals spheres compression, but more generally from electrostatic attraction forces and van der Waals repulsion, both of which are improper H-bonding components.
Y1  - 2011
U6  - https://doi.org/10.1021/jo102353f
SN  - 0022-3263
VL  - 76
IS  - 11
SP  - 4432
EP  - 4443
PB  - American Chemical Society
CY  - Washington
ER  -