Gas-Phase NMR Studies of N,N-Dimethylthioamides
J. Phys. Chem. A, Vol. 101, No. 26, 1997 4705
TABLE 6: Atomic Charges for the Ground-State Geometry Calculated Using Gaussian 94 (6-31+G*)
formamide
thioformamide
carbamyl fluoride
thiocarbamyl fluoride
carbamyl chloride
thiocarbamyl chloride
O ) -0.5921
C ) +0.4482
N ) -0.9028
H ) +0.1718
H ) +0.4478
H ) +0.4271
S ) -0.4097
C ) +0.0807
N ) -0.7909
H ) +0.2314
H ) +0.4550
H ) +0.4335
O ) -0.5774
C ) +1.050
N ) -1.007
F ) -0.3859
H ) +0.4624
H ) +0.4581
S ) -0.2363
C) +0.4568
N ) -0.8759
F ) -0.2756
H ) +0.4730
H ) +0.4581
O ) -0.4887
C ) +0.4592
N ) -0.8984
Cl ) +0.0110
H ) +0.4617
H ) +0.4551
S ) -0.3714
C ) +0.1556
N ) -0.7851
Cl ) +0.0744
H ) +0.4664
H ) +0.4600
interaction is between carbon and nitrogen, with carbon giving
up charge to nitrogen. During rotation, charge flows back from
the amide nitrogen to the carbonyl carbon. Electron-withdraw-
ing substituents on the (thio)formyl carbon would be expected
to decrease the charge that it can donate to the amide nitrogen.
When less charge is donated, rehybridizaton becomes less costly,
lowering the barrier. This is the same argument used to explain
the higher barriers in thioamides (over oxoamides) where the
“softer” thioformyl group was said to donate more charge than
the more polarized formyl group, raising the thioamide rotational
barrier by making rehybridization more costly.18 This argument
is not, however, compatible with the fact that electronegative
substituents raise the barrier in oxoamides or that the barrier in
DMCF is greater than in DMCCl.
in carbamylchloride and chlorine takes on a slight positve
charge, not inconsistent with contribution from previously
suggested resonance form III. In fact, the decrease in ∆Gq
298
due to chlorine substitution in thioamide systems may be a
combined steric and electronic effect. It is also interesting to
note that all of these changes have little effect on the amido
hydrogens, which have a positive charge of about half an
electron for all the molecules listed. Nitrogen’s negative charge
differs by a maximum of ca. 0.2 electron, having the greatest
negative charge in carbamyl fluoride and the least in thiofor-
mamide. Nitrogen has more negative charge in the oxoamides
than in the thioamides. The ab initio results are consistent with
the smaller effect of electron-withdrawing substituents on
thioamides and on the decrease in the rotational barrier when
CF3 is substituted for CH3 in DMTA. We hypothesize that the
smaller positive charge on the thiocarbonyl carbon renders the
mechanism responsible for destabilization of the transition state
much less important in thioamides.
Steric and electronic factors both influence C-N rotational
barriers of acetamides and trifluoroacetamides. In the trifluo-
roacetamides, the positive charge on the trifluoro carbon results
in a repulsive interaction with the positively charged carbonyl
In conclusion, these results suggest that the rotational barrier
in N,N-dimethylthioamides is extremely sensitive to the steric
features of the thiocarbonyl substituent. Furthermore, it is the
relative nonpolarity of the CdS moiety and the neutrality of
the thiocarbonyl carbon that are important in determining the
effect of electronegative substituents. Work in progress exam-
ines the effect of inductive substituents at both the carbonyl
(thiocarbonyl) and nitrogen positions in amides and thioamides.
carbon, which should destabilize the transition state and raise
2
∆Gq
.
Conversely, the CF3(4.4 Å) group is much larger than
298
the CH3(3.8 Å) group, which should destabilize the ground state
and lower ∆Gq
Previous gas-phase studies of dimethyla-
298
298.
mides found that ∆Gq is higher for DMCF3 than for DMA,
indicating that electronic effects are more important than steric
effects.35 The thioamides show a different trend. The ∆Gq
298
of DMTCF3 is less than that of DMTA. Steric repulsive forces
between CF3 and CdS should be greater than between CF3 and
CdO, and the lower barrier for DMTCF3 versus DMTA
indicates that steric effects predominate over electronegativity
effects in these molecules.
Acknowledgment. We are pleased to acknowledge the
National Science Foundation (CHE 93-21079) for support of
this research. C.B.L. also thanks the Idaho State Board of
Education for support from Specific Research Grant S96-086.
To further explore the changes in atomic charge and polarity
of the (thio)carbonyl bond when electron-withdrawing substit-
uents are present, we performed ab initio calculations at the
6-31+G* level on formamide, thioformamide, carbamyl fluo-
ride, thiocarbamyl fluoride, carbamyl chloride, and thiocarbam-
ylchloride. The atomic charges obtained for the geometry-
optimized ground states are given in Table 6. The results show
that the difference in atomic charge between the oxygen and
the carbon in formamide is considerably greater than the
difference in atomic charge between carbon and sulfur in
thioformamide. In fact, the carbon atom in the ground state of
thioformamide is essentially uncharged and the CdS bond is
relatively nonpolar when compared with the polar CdO bond
in amides. When the more electronegative fluorine atom is
substituted for hydrogen in formamide, little change occurs in
the atomic charge of the oxygen, but the carbonyl carbon is
more positive. In the sulfur analog, thiocarbamylfluoride, the
atomic charge of sulfur becomes less negative and carbon’s
positive charge increases. The polarity of the CdS bond in
thiocarbamylfluoride is considerably less than in carbamylfluo-
ride, but greater than in thioformamide. Fluorine may withdraw
more charge from an essentially uncharged thiocarbonyl adjacent
carbon than from a positively charged carbonyl carbon. In the
thioamide, this may stabilize the transition state where a full
CdS bond exists and decrease the rotational barrier. Substitu-
tion of chlorine for hydrogen has less effect on atomic charges
and polarity. Carbon suffers little increase in positive charge
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