Carbonyl Carbon Chemical Shift Tensors for a Typical Aryl Aldehyde and Formaldehyde. NMR Studies of the Isolated 13C-2H Spin Pair of 3,4-Dibenzyloxybenzaldehyde-13C.alpha.2H.alpha.

Abstract

The magnitude and orientation of the principal components of the carbonyl carbon chemical shift tensor of a typical aryl aldehyde, 3,4-diben~yloxybenzaldehyde-~~C,~H,, 1, have been determined from the analysis of I3C NMR spectra of static powder samples obtained at 4.7 and 9.4 T. The principal components of the carbon shielding tensor of formaldehyde, 2, have been calculated from the experimental I3C nuclear spinrotation tensor, which is available in the literature. The experimental results are compared with those of ab initio chemical shielding calculations using the GIAO (Gauge-Zncluding Atomic Orbitals) method. For 1, the most shielded component of the carbonyl carbon chemical shift tensor, 833, is perpendicular to the 2H--'3C=0 plane, whereas the least shielded component, 81 I , is within this plane approximately perpendicular to the C-0 bond. The orientation of the carbon shielding tensor of formaldehyde is analogous to that of 1; however, the precise orientation of the three principal components is dictated by the higher molecular symmetry (e.g., 8 2 2 is coincident with the C-0 bond). The deshielding associated with 811 is attributed to a large local magnetic dipole transition moment of the an* type. Analysis of I3C NMR spectra of solid 3,4-diben~yloxybenzaldehyde-'~C~~H, obtained with magic-angle spinning yields a value for the indirect 13C-'H spin-spin coupling constant, 1J(13C,2H)iSo = 26.4 f 0.5 Hz. From 2H NMR spectra of a static powder sample of 1, the 2H nuclear quadrupolar coupling constant was estimated to be 151.5 f 1 kHz. rotation data available for formaldehyde or the relationship between nuclear spin-rotation tensors and nuclear magnetic shielding tensors. Finally, we compare experimental carbonyl carbon shielding tensors of the aldehyde moiety to those calculated using the GIAO (Gauge-Including Atomic Orbitals) method. NMR Larmor frequency, yc is the I3C magnetogyric ratio, Bo

Natural chemical shielding analysis of nuclear magnetic resonance shielding tensors from gauge-including atomic orbital calculations

“…Stanton and Gauss 14 recently determined that treatment of electron correlation is required to obtain better agreement with experimental values of the carbon shielding, 30,51 shown in the last row of Table IV.…”

“…Because the n canonical MO has the single largest contribution to the shielding tensor component zz (C) and its ''rotated'' counterpart L z n has a very good overlap with the CO MO, there appears to be a reasonable theoretical justification for this assumption. It is of interest that Wu et al 30 used the IGLO analysis of the carbon shielding in H 2 CO to argue that the n→* excitation does not play a significant role in determining the value of zz (C), based on the observation that the IGLO contribution is positive, whereas a paramagnetic n→* magnetic excitation should give a negative contribution. However, NCS analysis finds that the n contribution to zz (C) negative in sign ͑Table IV͒, which is consistent with the n→* excitation model.…”

“…The IGLO method has been applied successfully to hydrocarbon chemical shieldings by Schindler and Kutzelnigg, 18 who proposed a theoretical increment system to predict proton and carbon chemical shifts from the number and type of neighboring bonds in a manner similar to existing empirical rules. 26,27 Other recent localized analyses of NMR shieldings in organic compounds include studies of conformational and substituent effects on the 13 C spectra of substituted butanes 28 and synthetic polymers, 29 isotropic and anisotropic 13 C shielding in carbonyl and thiocarbonyl compounds, 30,31 and the aromatic ''ring current'' effect. 32 The LORG program was successfully used to investigate the electronic origin of antisymmetry in carbon shielding tensors of cyclopropene and other three-membered ring compounds.…”

Applications and evaluation of IGAIM 13C and 1H chemical shift calculations for unsaturated hydrocarbons and organolithium compounds

“…IGAIM calculations yield the three principle components, which are averaged to obtain the isotropic shifts. Ideally, the IGAIM principle components should be compared with the experimental values if they are available (15,16 For personal use only. Table 4 is seen that in going from 5 to 6 the chemical shifts of the terminal carbons and the central carbon atom increase by 10.5 and 15.2 ppm, even though the electron populations of terminal carbons C 1 and C3 and the center carbon C2 increase by 0.067 and 0.101, respectively (Fig.…”

1H chemical shifts in NMR: Part 19. Carbonyl anisotropies and steric effects in aromatic aldehydes and ketones

“…Wasylishen and co-workers 14 used MASNMR to determine the chemical shift tensors for the carbonyl carbon of acetaldehyde, 3,4-dibenzyloxybenzaldehyde and 3,4-dimethoxybenzaldehyde. There was a significant difference (>25%) between the shift tensors for the carbonyl carbon of acetaldehyde and those for the carbonyl carbons of the benzaldehydes, which were identical (within the error margin of the observations).…”