DFT/FPT studies of the structural dependencies of long‐range ¹H,¹H coupling over four bonds ⁴J(H,H′) in propanic and allylic systems

Abstract

A study is presented of the structural dependencies for scalar J-coupling across four bonds in propanic and allylic systems. Density functional theory (DFT) and finite perturbation theory (FPT) were used to obtain the Fermi contact (FC) contributions to 1 H, 1 H coupling. Interproton couplings 4 J(H,H ) in propane were obtained as functions of the dihedral angles j 1 and j 3 , which are measured about the C(1) -C(2) and C(2) -C(3) bonds, respectively, and on the C(1) -C(2) -C(3) internal angle q 2 . A four-term trigonometric expression reproduces the dependence on j 1 and j 3 with a standard deviation of 0.1 Hz. The molecular structures for a series of bicycloalkanes were fully optimized and DFT/FPT results for 4 J(H,H ) are in good agreement with the available experimental data. However, in non-W arrangements of bicyclic systems the propane trigonometric expression for 4 J(H,H ) often gives signs opposite to those observed. It is now clear the conformational dependence of coupling constants in propane is not, in general, applicable to bicyclic systems. The DFT/FPT propane-based results for coupling involving a methyl group (H -C -C -CH 3 ) are in better conformity with the experimental data. The DFT/FPT results for cisoid and transoid allylic coupling 4 J(H,H ) in propene were obtained as functions of the dihedral angle j measured about the C(2) -C(3) bond. Trigonometric expressions with three terms reproduce the calculatedpropene results with standard deviations 0.1 Hz. The molecular structures for a series of bicycloalkenes were fully optimized and DFT/FPT results for cisoid and transoid coupling are in fairly good agreement with the available experimental data. In contrast to the propanic coupling, the trigonometric expressions for propenic coupling constants are generally applicable to conformational analyses in bicycloalkenes.

First observable CD spectra from n-σ* excitation: TD-DFT calculation and determination of absolute configuration of 2,6-dithiaspiro[3.3]heptane 2,6-dioxide

“…This is because of the acute bond angle of 94.7° for C1-C4-C3 and the large dihedral angles of 145.2° for H1eq-C1-C4-C3 and -145.2° for C1-C4-C3-H3eq; the other values are too small to observe using a 400 MHz NMR instrument. 18 This observation is in quite contrast of 1 H NMR of 3, which showed only one averaged singlet in toluene-d 8 even at -70 °C. It is because the racemization barrier (inversion barrier of the thietane ring) was calculated to be 1.1 kcal/mol at the BMK/6-311+G(d,p)//B3LYP/6-31G(d) level.…”

DFT studies of the conformational/structural dependencies of geminal 1H,1H scalar coupling 2J(H,H′) in substituted methanes

“…These interactions are thought to be responsible for the differences between the exo-exo and endo-endo vicinal coupling constants in bicycloalkanes. 67 Also, in a previous study of long range H-H coupling, 33 it was concluded that trigonometric equations applicable to propane coupling could not account for many features of 4 J(H,H) in the cyclic compounds.…”

“…The solid line is a plot of the linear regression result from Eqn (34). The squares depict the aug-cc-pV(D)Z-J results from Table 2 scaled according to Eqn (33). The scaling is used only to show that the computed results include all of the subtle features of coupling in the bicyclic molecules.…”

“…Also plotted (as squares) are the calculated (CP/DFT/augcc-pVT(D)-J) values from column 9. The DFT results are scaled according to Eqn (33) to show that all the trends in the experimental data are accurately reproduced. The monotonic increase of the calculated and experimental data with increasing H-C-H angle is consistent with the linear Figure 10.…”

“…Because HTOs can be written in terms of internal and dihedral angles, it is often possible to relate the matrix elements and, therefore, coupling constants to angles and internuclear separations. Such models have been used in these laboratories to obtain explicit expressions in terms of the structural factors such as the H-C-C internal angle dependence of 3 J H,H 0 ; 32 the dependence of long range coupling 4 J H,H 0 over four bonds on dihedral angle and internal angles; 33 and the relationships of trans H-bonding J-coupling to H-bond geometries in nucleic acids and proteins. 28,29 In the next sections, an LCAO-MO/SOS analysis will be used to examine the structural dependencies of 2 J H,H 0 in a CH 2 moiety, Figure 1.…”