A solid-state ¹³C NMR and theoretical investigation of carbonyl and thiocarbonyl carbon chemical shift tensors

Abstract

The carbon chemical shift tensors of the carbonyl and thiocarbonyl groups of acetamide, thioacetamide, thioacetanilide, 4'-methoxyacetanilide, and 4'-methoxythioacetanilide have been experimentally determined using dipolar -chemical shift solid-state 13c NMR spectroscopy. The magnitudes of the three principal components of the carbon chemical shift tensors are found to exhibit marked variations between the carbonyl and thiocarbonyl functionalities. However, in contrast to the conclusions of an earlier comparative investigation involving benzophenone and thiobenzophenone, the orientations of the principal axis systems of these chemical shift tensors are found to be similar. These experimental results represent the first complete characterizations of the carbon chemical shift tensor in organic thiocarbonyls. The results of our ab initio GIAO and LORG calculations of carbon chemical shielding tensors in formaldehyde, thiofomaldehyde, formamide, and thioformamide as well as in acetamide and thioacetamide are in qualitative agreement with experiment. The findings of the present investigation provide conclusive evidence that the well-known isotropic deshielding of the carbon nucleus in the C=S group relative to C=O is primarily attributable to the decreased energy associated with the o tt T* excitation within the thiocarbonyl fragment. This result is in contrast with the conventional interpretation that the deshielding originates from a red shift in the C=S HOMO-LUMO n + T* transition.Key words: chemical shift tensors, solid-state I3C NMR, carbonyls, thiocarbonyls, ab initio calculations.RksumC : Utilisant les deplacements chimiques dipolaires de la spectroscopie RMN du I3c 2i I'ttat solide, on a determine exptrimentalement les tenseurs des dtplacements chimiques du carbone des groupes carbonyle et thiocarbonyle des acttamide, thioacetamide, thioacttanilide, 4'-mCthoxyacCtanilide et 4'-mCthoxythioacttanilide. On a trouvt que les amplitudes des trois principales composantes des tenseurs des dtplacements chimiques du carbone presentent des variations importantes entre les fonctions carbonyles et thiocarbonyles. Toutefois, en opposition avec les conclusions tirkes 2i partir d'une ttude comparative anttrieure impliquant la benzophenone et la thiobenzophtnone, on a trouvt que les orientations des principaux axes de rtftrence de ces tenseurs des dCsplacements chimiques sont semblables. Ces resultats exptrimentaux representent les premihres caractkrisations complktes du tenseur du dtplacement chimique des thiocarbonyles organiques. Les rtsultats de nos calculs ab initio GIAO et LORG des tenseurs d'tcran chimique du carbone dans les formaldthyde, thiofomaldChyde, formamide, thiofomamide ainsi qu'acttamide et thioacttamide sont en accord qualitatif avec les rtsultats expCrimentaux. Les observations faites dans le present travail fournissent des bases concluantes qui permettent d'attribuer le deblindage bien connu du noyau du carbone dans le groupe C=S par rapport au C=O 2i une diminution de l'energie associee B une e...

The 13C Chemical Shift Tensor Principal Values and Orientations in Dialkyl Carbonates and Trithiocarbonates

“…The experimental 13 The principal values given in Table 1 correlate qualitatively with the principal values measured by Kirby et al [8] for the trigonal carbon atoms in acetamide, acetanilide, 4 0 -methoxyacetanilide, and the corresponding thiocarbonyl derivatives. These authors found that each of the three principal values for the carbonyl carbon atoms reveals a significant sensitivity to a substitution of the carbonyl oxygen with sulfur.…”

“…The principal values and tensor orientation data obtained for EC, ETTC, DPC, ETTC, and DMTTC in this study complement the previous data on the ionic potassium carbonates and thiocarbonates [1], the PAHs [7,[32][33][34][35][36][37], and the C/TC compounds studied by Kirby et al [8]. Figure 5 compares the bonding structure of the planar carbonate and thiocarbonate groups in the neutral dialkyl compounds with the bonding structure of the planar groups in the monoalkyl carbonate and thiocarbonate anions.…”

“…The 13 C principal values and tensor orientations obtained for EC, ETTC, DPC, DMC, and DMTTC in this study are in agreement with the trends found in the previous study. Furthermore, these new chemical shift tensor data qualitatively correlate with the results obtained by Kirby et al [8] for a series of carbonyl and thiocarbonyl (C/TC) compounds and present further experimental evidence for the suggested rules that predict the chemical shift tensor principal values and orientations for planar trigonal carbon atoms in various molecular environments.…”

“…These unique bonds are shorter and higher in bond order than the two adjacent formal single bonds, which are similar in bond lengths and bond orders. According to the rules for chemical shift tensor orientations established for PAHs [7] and C/TC compounds [8], this change in character of the unique bonds was expected to result in d 11 being the d p component in the dialkyl carbonates and trithiocarbonates studied here. This predicted trend is observed in Fig.…”

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.…”