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Ab initio SCF study of the barrier to internal rotation in simple amides. Part 3. Thioamides
Journal of Molecular Structure 654 (2003) 27–34 www.elsevier.com/locate/molstruc
Ab initio SCF study of the barrier to internal rotation in simple amides. Part 3. Thioamides Nikolay G. Vassilev, Valentin S. Dimitrov* Institute of Organic Chemistry, Bulgarian Academy of Sciences, Sofia 1113, Bulgaria Received 23 December 2002; accepted 10 February 2003
Abstract The free energies of activation for rotation about the thiocarbonyl C– N bond in X– C(S)N(CH3)2 (X ¼ H, F, Cl, CH3, CF3) were calculated at the MP2(fc)/6-31 þ G*//6-31G* and MP2(fc)/6-311þþ G**//6-311þ þG** levels and compared with literature NMR gas-phase data. The results of calculations indicate that the nonbonded interactions in ground state (GS) are mainly responsible for the differences in the rotational barriers. For X ¼ H, CH3 and CF3, the anti transition state (TS) is more stable; for the case X ¼ Cl, the syn TS is more stable, while for the X ¼ F, the two TS are energetically almost equivalent. q 2003 Elsevier Science B.V. All rights reserved. Keywords: Thioamides; Barrier to internal rotation; Ab initio SCF study
1. Introduction Amides are simple models for the peptide bond in proteins. The internal rotation about the amide C –N bond in amides and thioamides have been intensively studied experimentally by NMR spectroscopy in the gas [1 –7] and in the liquid phase [8]. The experimental results were used to judge theoretical methods of calculating barrier heights. The origin of the C –N rotational barrier and its relation to amide resonance have also received much attention in the last years [9 – 14]. We were interested more in the origin of the differences in the rotational barriers and in Part 1 of this series [15] we found that in case of * Corresponding author. Tel.: þ359-2-9606-189; fax: þ 359-28700-225. E-mail address: [email protected] (V.S. Dimitrov).
X –C(O)N(CH3)2 (X ¼ H, F, Cl and Br) the repulsion between X and methyl group in ground state (GS) and the repulsion between X or oxygen and nitrogen lone pair in transition states (TS) are largely responsible for the difference in the free energies of the studied amides. In Part 2 of this series [16] we conclude that in case of R – C(O)N(CH3)2 (R ¼ CH3, CH2F, CHF2, CF3 and CCl3) the repulsion between the substituent R and the methyl group in GS and the conformation of R itself are mainly responsible for the difference in the free energies of the studied amides. We have continued this study by ab initio calculations of rotational barriers of thioamides: N; N-dimethylthioformamide (DMTF), N; Ndimethylthiocarbamyl fluoride (DMTCF), N; N-dimethylthiocarbamyl chloride (DMTCCl), N; N-dimethylthioacetamide (DMTA) and N; Ndimethyltrifluorothioacetamide (DMTFTA).
0022-2860/03/$ - see front matter q 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0022-2860(03)00177-7
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N.G. Vassilev, V.S. Dimitrov / Journal of Molecular Structure 654 (2003) 27–34
2. Methods The ab initio SCF calculations were performed using the GAMESS package [17]. The complete geometry optimization was carried out using the 631G* and 6-311þ þ G** basis sets, which have been shown to provide excellent results for the structures of neutral molecules [18]. The molecules were assumed to have C1 symmetry in the GS and CS symmetry in the TSs (Fig. 1). The energies were then calculated at the MP2(fc)/6-31 þ G*//6-31G* and at the MP2(fc)/ 6-311þ þ G**//6-311þ þ G** levels of theory. The Hessians were calculated numerically with the 631G* basis set using HF/6-31G* optimized geometry. In calculating vibrational energies, the vibrational frequencies were scaled by a factor of 0.89 [18]. In each case, seven (for the TS) or eight (for the GS) scaled frequencies below 500 cm21 were treated as rotations ðE ¼ RT=2Þ [18]. The imaginary frequency for the TSs is ignored in all thermodynamics calculations.
3. Results and discussion 3.1. Geometry optimization The heavy-atom framework of the studied thioamides in the GS was found to be essentially planar and very close to CS symmetry. The calculated energies are presented in Table 1 and the selected geometry parameters appear in Table 2. The substitution of H atom in DMTF with halogen atom shortens the CyS ˚ while the same substitution bond with less then 0.01 A
˚. in oxoamides shortens the CyO bond with 0.02 A This suggests that the CyS group is less affected by electron withdrawing or electron donating substituents and in thioamides nonbonded interactions will be more important. Therefore we wish to single out the conformation of the methyl groups syn and anti-to the thiocarbonyl group (Fig. 1). In all compounds the dihedral angle C2 – N4 – C5 –H7 is about 608, while in the case of DMTF this dihedral angle is about 08. This suggests that the S· · ·H nonbonded repulsion is significant and the syn methyl groups are rotated in order not to be eclipsed with the sulfur atom in contrast to oxoamides in which the methyl hydrogens are almost eclipsed with the carbonyl oxygen [15]. Recently the MP2/6-311 þ G** ab initio calculations of DMTF [14] result in structure with dihedral angle C2 –N4 – C5 –H7 of about 608. In all compounds the dihedral angle C2 – N4 – C6 –H10 is about 08, while in the case of DMTCClF this dihedral angle is about 608. This suggests that in case of DMTCClF the repulsion between chlorine and anti methyl group is significant and the anti methyl group is rotated in order not to be eclipsed with the chlorine. Similar is the conformation of anti methyl groups in oxoamides X –C(O)N(CH3)2 (X ¼ Cl and Br), in which the repulsion interaction between halogen atoms and anti methyl group is responsible for increasing the energy of the GSs and lowering of the C – N rotational barrier [14]. The calculated dipole moment of DMTF (5.77 and 5.82 D, Table 2) is in good agreement with the previously observed value (4.74 D) [19]. The TS geometries of DMTF, DMTCF, DMTCCl, DMTA, DMTFTA (Fig. 1) were optimized in CS symmetry. The calculated energies, selected geometry
Fig. 1. Conformations and numbering of the studied thioamides (X ¼ H, F, Cl, CH3, CF3).
N.G. Vassilev, V.S. Dimitrov / Journal of Molecular Structure 654 (2003) 27–34
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Table 1 Calculated energies for the studied thioamides X –C(S)NR2 in Hartrees X
R
State
ZPEa
HF/6-31G*
MP2(fc)/6-31 þ G*//6-31G*
HF/6-311þ þG**
MP2(fc)/6-311þ þG**// 6-311þ þ G**
H H H F F F Cl Cl Cl CH3 CH3 CH3 CF3 CF3 CF3 H H H F F F Cl Cl Cl CH3 CH3 CH3 CF3 CF3 CF3
CH3 CH3 CH3 CH3 CH3 CH3 CH3 CH3 CH3 CH3 CH3 CH3 CH3 CH3 CH3 H H H H H H H H H H H H H H H
GS anti TS syn TS GS anti TS syn TS GS anti TS syn TS GS anti TS syn TS GS anti TS syn TS GS anti TS syn TS GS anti TS syn TS GS anti TS syn TS GS anti TS syn TS GS anti TS syn TS
60.32 59.53 59.63 55.81 55.40 55.32 54.88 54.33 54.26 77.19 76.45 76.29 63.93 63.28 63.13 26.50 25.74 25.53 21.99 21.46 21.26 21.00 20.42 20.33 43.17 42.54 42.36 30.14 29.37 29.26
2569.626326 2569.588497b 2569.587683 2668.483936 2668.451205 2668.451901b 21028.516442 21028.489492 21028.490245b 2608.660989 2608.632606b 2608.626562 2905.228680 2905.204307b 2905.197218 2491.566677 2491.532598b 2491.528849 2590.425864 2590.394101b 2590.393320 2950.463266 2950.431922b 2950.431794 2530.609163 2530.577775b 2530.571258 2827.177960 2827.147050b 2827.144978
2570.310144 2570.276525b 2570.275910 2669.345169 2669.318980b 2669.318863 21029.341586 21029.317808 21029.319192b 2609.481063 2609.456369b 2609.449920 2906.578808 2906.554055b 2906.546301 2491.984423 2491.953768b 2491.951126 2591.021058 2590.994454 2590.995099b 2951.018256 2950.990606 2950.991442b 2531.159355 2531.132445b 2531.127274 2828.257055 2828.228660b 2828.227499
2569.697307 2569.660418b 2569.659430 2668.584573 2668.553968b 2668.553616 21028.615270 21028.588901 21028.589854b 2608.741696 2608.714040b 2608.708098 2905.394909 2905.371524b 2905.363904 2491.624113 2491.591121b 2491.588137 2590.513396 2590.483482b 2590.482418 2950.548601 2950.518016 2950.518217b 2530.676304 2530.645941b 2530.640017 2827.330172 2827.301222b 2827.299363
2570.458973 2570.426802b 2570.426201 2669.549753 2669.524987b 2669.524122 21029.521700 21029.498513 21029.500024b 2609.660446 2609.636037b 2609.629925 2906.924248 2906.900232b 2906.891974 2492.078396 2492.049365b 2492.047338 2591.171236 2591.146327 2591.146613b 2951.142821 2951.116584 2951.117674b 2531.283022 2531.257572b 2531.253009 2828.547623 2828.520395b 2828.519227
a b
ZPE is reported in kcal/mol at the HF/6-31G* level scaled by 0.89. More stable TS.
parameters for anti and syn TSs, are presented in Tables 1, 3 and 4, respectively. The anti TS is more stable in the cases of DMTF, DMTA and DMTFTA, in the case of DMTCCl the synTS is more stable, while in the case of DMTCF the two TS are energetically almost equivalent. The most significant structural changes in the process of rotation towards the TSs are that the nitrogen is pyramidalized, the C – N bond lengthens ˚ and the CyS bond from 1.32– 1.33 to 1.39 –1.43 A ˚ . However, shortens from 1.65 –1.67 to 1.60 –1.62 A the CyO bond length in corresponding amides ˚ only. This indicates that shortens by 0.01– 0.02 A the thiocarbonyl group is relatively more affected by
this rotation in comparison to carbonyl group in the oxoamides. 3.2. Comparison of calculated activation parameters and experimental data Calculation of the vibrational frequencies confirmed the assignment of the anti and syn forms as TSs and allowed computation of the enthalpy, entropy and free energy changes at 298 K. The thermodynamic results for the isomerisation of the studied amides are presented in Table 5. DH – (298 K) is obtained from the sum of the changes in the electronic energy, DEeo ; the zero-point vibrational energy, DEvo ; and
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Table 2 Selected structural parameters and dipole moments (m) for GS of the studied thioamides Parameters Bond lengths S1 –C2 C2 –X3 C2 –N4 N4–C5 N4–C6 S1 –H7 Bond angles S1 –C2–N4 X3–C2 –N4 C2 –N4–C5 C2 –N4–C6 Dihedral angles C2 –N4–C5– H7 C2 –N4–C6– H10 S1 –C2–N4– C5 X3–C2 –N4–C6 m
X¼H
1.653(1.654) 1.078(1.080) 1.321(1.319) 1.452(1.453) 1.451(1.452) 2.649(2.644)
X¼F
1.648(1.646) 1.314(1.312) 1.317(1.316) 1.459(1.460) 1.459(1.461) 2.996(2.996)
X ¼ Cl
1.648(1.647) 1.752(1.756) 1.324(1.322) 1.465(1.466) 1.465(1.466) 2.910(2.917)
X ¼ CH3
1.670(1.670) 1.515(1.515) 1.329(1.328) 1.459(1.459) 1.455(1.456) 2.902(2.904)
X ¼ CF3
1.651(1.649) 1.539(1.543) 1.324(1.323) 1.461(1.642) 1.464(1.645) 2.890(2.894)
129.5(129.5) 112.2(112.6) 123.4(123.3) 121.4(121.2)
128.5(128.6) 112.1(112.2) 119.5(119.4) 123.6(123.8)
126.8(127.0) 114.2(114.2) 117.9(118.0) 123.5(123.6)
123.0(123.0) 117.8(117.9) 120.3(120.0) 124.9(124.8)
125.1(125.2) 116.2(116.3) 118.6(118.6) 127.4(127.4)
0.0(0.0) 0.0(0.0) 0.0(0.0) 0.0(0.0) 5.77(5.82)
60.2(60.2) 0.0(0.0) 0.0(0.0) 0.0(0.0) 5.73(5.73)
59.9(59.9) 60.8(60.8) 0.0(0.0) 0.0(0.0) 5.52(5.51)
60.1(60.1) 20.3(20.3) 0.0(0.0) 0.0(0.0) 5.76(5.79)
59.5(59.9) 0.2(0.2) 0.0(0.0) 0.0(0.00) 5.28(5.35)
The parameters are given for HF/6-31G* optimized geometry while the values in parenthesis correspond to HF/6-311þ þG** optimized ˚ and bond angles are given in deg. Dipole moments are given in Debye. For numbering of atoms see Fig. 1. geometry. Bond lengths are given in A
the thermal correction to the zero-point energy, DDEv298 : The scaled frequencies were used also for the entropy calculation. The two lowest real frequencies, which correspond to rotations of the methyl groups,
need to be treated as hindered rotations. The resultant free energy DG– (298 K) for the anti TS (DMTF, DMTA and DMTFTA) and for the syn TS (DMTCCl) should be compared with the gas-phase NMR results.
Table 3 Selected structural parameters and dipole moments ðmÞ for anti TS of the studied thioamides Parameters Bond lengths S1 –C2 C2 –X3 C2 –N4 N4–C5(N4–C6) Bond angles S1 –C2–X3 S1 –C2–N4 C2 –N4–C5 (C2– N4– C6) Dihedral angles S1 –C2–N4– C5 X3–C2 –N4–C6 m
X¼H
1.608(1.607) 1.080(1.082) 1.419(1.419) 1.458(1.459)
X¼F
1.609(1.606) 1.307(1.306) 1.395(1.393) 1.461(1.462)
X ¼ Cl
1.605(1.604) 1.743(1.748) 1.403(1.402) 1.459(1.460)
X ¼ CH3
1.619(1.619) 1.500(1.499) 1.429(1.429) 1.457(1.458)
X ¼ CF3
1.599(1.598) 1.527(1.532) 1.414(1.413) 1.459(1.460)
119.2(119.1) 127.6(127.5) 113.0(113.0)
120.2(120.0) 129.1(129.2) 113.4(113.3)
121.0(120.7) 127.3(127.4) 113.3(113.2)
123.6(123.4) 124.5(124.3) 113.0(113.0)
122.5(122.1) 127.4(127.5) 113.4(113.3)
264.6(264.7) 2115.4(2115.3) 2.10(2.10)
265.4(265.4) 2114.6(2114.6) 3.06(3.04)
265.7(265.6) 2114.3(2114.4) 2.69(2.69)
264.4(264.4) 2115.6(2115.6) 2.47(2.44)
265.5(265.3 2114.5(2114.7) 2.72(2.96)
The parameters are given for HF/6-31G* optimized geometry while the values in parenthesis correspond to HF/6-311þ þG** ˚ and bond angles are given in deg. Dipole moments are given in Debye. For numbering of optimized geometry. Bond lengths are given in A atoms see Fig. 1.
N.G. Vassilev, V.S. Dimitrov / Journal of Molecular Structure 654 (2003) 27–34
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Table 4 Selected structural parameters and dipole moments (m) for syn TS of the studied thioamides Parameters Bond lengths S1 –C2 C2 –X3 C2 –N4 N4–C5(N4–C6) Bond angles S1 –C2–X3 S1 –C2–N4 C2 –N4–C5 (C2–N4–C6) Dihedral angles S1 –C2–N4–C5 X3–C2 –N4–C6 m
X¼H
1.600(1.599) 1.087(1.089) 1.413(1.412) 1.457(1.458)
X¼F
1.600(1.598) 1.323(1.332) 1.390(1.389) 1.462(1.463)
X ¼ Cl
1.597(1.596) 1.774(1.778) 1.393(1.393) 1.458(1.459)
X ¼ CH3
1.613(1.613) 1.511(1.510) 1.426(1.426) 1.453(1.454)
X ¼ CF3
1.598(1.596) 1.535(1.540) 1.408(1.408) 1.455(1.457)
119.5(119.2) 125.5(125.7) 110.6(110.5)
120.2(119.9) 127.7(127.7) 112.5(112.7)
120.0(119.8) 125.4(125.7) 114.3(114.1)
122.3(122.2) 120.6(120.5) 114.1(113.8)
120.7(120.3) 123.3(123.4) 117.2(117.2)
2117.5(2117.6) 262.5(262.4) 3.42(3.41)
2115.3(2115.0) 264.7(265.0) 3.38(3.30)
2113.0(2113.2) 267.0(266.8) 3.00(2.98)
2113.7(2114.0) 266.3(266.0) 3.80(3.81)
2108.6(2108.6) 271.4(271.4) 2.39(2.52)
The parameters are given for HF/6-31G* optimized geometry while the values in parenthesis correspond to HF/6-311þ þG** ˚ and bond angles are given in deg. Dipole moments are given in Debye. For numbering of optimized geometry. Bond lengths are given in A atoms see Fig. 1.
In the case of DMTCF the two TS have similar energies and the resultant effective free energy DG– eff (298 K) takes into account the two possible routes and it is calculated by summing the rates through the two possible TS. It is seen from Table 5 that the barriers were calculated very satisfactorily. More precise results for the rotational barrier in amides and thioamides can be obtained using some of the composite methods like G2MP2 [16]. 3.3. Effect of the substituents on the rotational barrier It is worth to note that the experimental rotational barriers of the studied compounds X – C(S)N(CH3)2 in the gas phase follow the trend: H . F . CH3 . CF3 . Cl and correlate mainly with the substituent size and not with the substituent electronegativity. In order to find the reason for this tendency we examined the following reaction both in the GS and in the two possible TSs: H – C(S) – N(CH3)2 þ X – C(S) – NH2 ! H – C(S) – NH2 þ X – C(S) –N(CH3)2 The calculated enthalpy of the reaction for the GS, DDH 0 (GS), will be an estimate for the difference in repulsion between X and the CH3 group. The calculated DDH 0 (GS) values are the highest values
in Table 6 and therefore the repulsion between X and the anti CH3 group is mainly responsible for the differences in the rotational barriers in thioamides. The computed enthalpy of the reaction for the anti TS DDH 0 (anti TS) will be an estimate for the difference in repulsion between X and the thioamide lone pair, while the calculated enthalpy of the reaction for syn TS DDH 0 (syn TS) will be an assessment for the difference in repulsion between sulphur and thioamide lone pair. The DDH 0 (anti TS) values are very small and close to zero which means that there is no significant difference in the repulsion between X and amide lone pair in all studied compounds. The DDH 0 (syn TS) values are very small and close to zero in the case of X ¼ F and Cl, which indicate that the repulsion between X and amide lone pair is not important for restricted rotation in these compounds (DMTCF and DMTCCl). The combined energy differences for both GS and anti TS [DDH 0 (anti TS) 2 DDH 0 (GS)] should be compared with the calculated differences in the computed enthalpy of activation DDH – (0 K) for the anti TS, while the combined energy differences for both GS and syn TS ½DDH 0 (syn TS)2DDH 0 (GS)] should be compared with the calculated differences in the calculated enthalpy of activation DDH – (0 K) for the syn TS. [DDH 0 (anti TS)2DDH 0 (GS)] and [DDH 0 (syn TS)2DDH 0 (GS)] can also be compared with
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Table 5 Calculated barriers of the studied thioamides X –C(S)N(CH3)2 in the gas phase X
Methoda
TS
DH – (0 K)
DH – (298 K)
DS– (298 K)
DG– (298 K)
DG– eff (298 K)
H
1
anti syn anti syn anti syn anti syn
20.3 20.8 22.4 23.1 20.2 20.6 21.3 21.2
21.3
16.0 16.0 18.8 18.9 15.1 15.6
anti syn anti syn anti syn anti syn
14.8 18.6 16.6 20.2 14.6 18.2 16.9 19.5
anti syn anti syn anti syn
18.6 20.1 20.0 21.8 17.6 18.7
anti syn anti syn anti syn
14.4 13.4 16.0 15.3 14.0 13.0
28.9 28.7 28.9 28.7 28.9 28.7 25.3 25.6 20.7 ^ 2.5 27.0 26.8 27.0 26.8 27.0 26.8 23.0 ^ 3.0 26.4 25.1 26.4 25.1 26.4 25.1 25.6 24.8 23.3 ^ 2.0 26.3 24.7 26.3 24.7 26.3 24.7 20.1 ^ 3.0 25.8 25.6 25.8 25.6 25.8 25.6 20.2 ^ 4.0
21.5 22.1 23.6 24.4 22.0 22.5 22.6 22.5
anti syn anti syn anti syn
18.9 19.5 20.9 21.8 19.4 19.9 20.7 20.6 22.3 ^ 1.5 14.9 14.9 17.7 17.8 14.0 14.4 17.2 ^ 1.1 13.4 17.2 15.2 18.8 13.2 16.8 15.4 18.9 17.1 ^ 1.0 17.4 18.8 18.8 20.5 16.4 17.4 17.2 ^ 1.1 13.2 12.2 14.8 14.1 12.8 11.8 16.8 ^ 1.3
2 3 4 Exptlb 1
F
2 3
CH3
Exptlb 1 2 3 4
CF3
Exptlb 1 2 3
Cl
Exptlb 1 2 3 Exptlb
a b
17.0 16.9 19.8 19.8 16.1 16.5 15.4 19.3 17.3 20.8 15.3 18.9 17.4 20.5 19.5 20.9 20.9 22.6 18.4 19.5 15.3 14.3 16.9 16.2 14.9 13.8
23.4 21.8 21.9 22.5 ^ 0.1 16.5 19.4 15.8 18.3 ^ 0.1 15.4 17.3 15.3 17.3 18.0 ^ 0.1 19.4 20.9 18.4 17.2 ^ 0.1 14.2 16.0 13.7 16.9 ^ 0.1
DG– eff (298 K) is calculated by summing up the rates through the two possible TS. (1) MP2(fc)/6-31 þ G*//6-31G*, (2) HF/6-311þ þ G**, (3) MP2(fc)/6-311þþ G**//6-311þþ G**, (4) G2MP2 at 353 K from Ref. [12]. Ref. [7].
the calculated change in the free energy DDG– eff (298 K) and with the experimental change in the free energy of activation DDG– exp (298 K). Since the anti TS is the preferred one in the cases of DMTA and DMTFTA, the values in the row [DDH 0 (anti TS) 2 DDH 0 (GS)] should be compared with energy – differences DDG– eff (298 K) and DDGexp (298 K). It is
seen from Table 6 that the calculated [DDH 0 (anti TS) 2 DDH 0 (GS)] contributions are of the order of magnitude of the experimental energy differences DDG– exp (298 K). In the case of DMTCCl the synTS is more stable and therefore the values in the row [DDH 0 (syn TS) 2 DDH 0 (GS)] should be compared with energy differences DDG– and eff (298 K)
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Table 6 Origin of the difference in the rotational barriers (kcal/mol) of the studied thioamides X– C(S)N(CH3)2 in the gas phase. H – C(S)– N(CH3)2 þ X– C(S)–NH2 ! H –C(S)–NH2 þ X –C(S)–N(CH3)2 X
H
F
DDH 0 (GS) DDH 0 (anti TS) DDH 0 (syn TS) anti TS –GS: DDH – (0 K) DDH 0 (anti TS)–DDH 0 (GS) syn TS –GS: DDH – (0 K) DDH 0 (syn TS)–DDH 0 (GS) DDG– eff (298 K) DDG– exp (298 K)
0(0) 0(0) 0(0)
1.0(1.3) 20.6(20.6) 0.1(0.0)
0(0) 0(0) 0(0) 0(0) 0(0) 0
CH3
CF3
Cl
5.1(5.1) 0.8(0.9) 2.0(1.8)
5.6(5.3) 20.6(20.4) 3.9(4.0)
4.1(4.1) 20.9(20.9) 0.1(20.4)
24.3(23.6) 21.6(21.9)
25.5(25.8) 24.3(24.2)
21.7(22.4) 26.2(25.7)
25.9(26.4) 25.0(25.1)
24.8(24.1) 20.9(21.3) 24.8(24.0) 24.2
22.2(22.9) 23.1(23.3) 25.9(26.1) 24.5
20.7(21.3) 21.7(21.3) 21.9(22.5) 25.3
27.4(27.8) 24.0(24.6) 27.1(27.4) 25.6
DDH 0 (GS), DDH 0 (anti TS) and DDH 0 (syn TS) are the energy changes (HF/6-31G* energy with ZPE correction) for the model reaction in the ground, anti transition and syn TSs, respectively. DDH – (0 K) is the difference in the calculated enthalpy of activation for the amide rotation, – DDG– eff (298 K) is the change in the calculated free energy and DDGexp (298 K) is the experimental change in free energy. The values in parenthesis correspond to the HF/6-311þ þG** level of theory.
DDG– exp (298 K). Again the calculated energy contributions [DDH 0 (syn TS) 2 DDH 0 (GS)] are similar to the experimental energy differences DDG– exp (298 K). In the case of DMTCF the two TS are energetically almost equivalent and the values in both rows [DDH 0 (anti TS) 2 DDH 0 (GS)] and [DDH 0 (syn TS) 2 DDH o (GS)] should be compared with the two energy differences DDG– and eff (298 K) – DDGexp (298 K). As it is seen from Table 6 these contributions to the experimental energy difference DDG– exp (298 K) depend on the level of calculation and are between 20% and 45%.
4. Conclusion The results of our calculations indicate that the nonbonded interactions (mainly the repulsion between X and anti CH3 in GS) are responsible for the differences in the free energies of rotation of the studied thioamides. This conclusion is based on the following facts: 1. The CyS group is less affected by electron withdrawing or electron donating substituents, as compared to the CyO group in oxoamides. 2. The S· · ·H nonbonded repulsion is significant and the syn methyl groups are rotated in order not to be eclipsed with the sulfur atom in comparison to
oxoamides in which the methyl hydrogen is almost eclipsed with the carbonyl oxygen. 3. The calculated DDH 0 (GS) values in the model reaction are the highest values in Table 6 and therefore the repulsion between X and the anti CH3 group is mainly responsible for the differences in the rotational barriers in thioamides.
References [1] C.B. LeMaster, Prog. Nucl. Magn. Res. Spectrosc. 31 (1997) 119. [2] A.N. Taha, N.S. True, J. Phys. Chem. A 104 (2000) 2985. [3] A.N. Taha, S.M. Neugebauer-Crawford, N.S. True, J. Phys. Chem. A 104 (2000) 7957. [4] C.L. LeMaster, C.B. LeMaster, N.S. True, J. Am. Chem. Soc. 121 (1999) 4478. [5] A.N. Taha, S.M.N. Crawford, N.S. True, J. Am. Chem. Soc. 120 (1998) 1934. [6] A.N. Taha, S.M.N. Crawford, N.S. True, J. Phys. Chem. A 102 (1998) 1425. [7] S.M.N. Crawford, A.N. Taha, N.S. True, C.B. LeMaster, J. Phys. Chem. A 101 (1997) 4699. [8] W.E. Stewart, T.H. Siddall III, Chem. Rev. 70 (1970) 517. [9] K.B. Wiberg, K.E. Laidig, J. Am. Chem. Soc. 109 (1987) 5935. [10] M.W. Wong, K.B. Wiberg, J. Phys. Chem. 96 (1992) 668. [11] K.B. Wiberg, P.R. Rablen, J. Am. Chem. Soc. 117 (1995) 2201. [12] K.B. Wiberg, P.R. Rablen, D.J. Rush, T.A. Keith, J. Am. Chem. Soc. 117 (1995) 4261.
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[13] K.B. Wiberg, D.J. Rush, J. Am. Chem. Soc. 123 (2001) 2038. [14] K.B. Wiberg, D.J. Rush, J. Org. Chem. 67 (2002) 826. [15] N.G. Vassilev, V.S. Dimitrov, J. Mol. Struct. 484 (1999) 39. [16] N.G. Vassilev, V.S. Dimitrov, J. Mol. Struct. 522 (2000) 37.
[17] M.W. Schmidt, K.K. Baldridge, J.A. Boatz, S.T. Elbert, M.S. Gordon, J.H. Jensen, S. Koseki, N. Matsunaga, K.A. Nguyen, S.J. Su, T.L. Windus, M. Dupuis, J.A. Montgomery, J. Comput. Chem. 14 (1993) 1347. [18] W.J. Hehre, L. Radom, P.v.R. Schleyer, J.A. Pople, Ab Initio Molecular Orbital Theory, Wiley, New York, 1986. [19] W. Walter, H. Hu¨hnerfuss, J. Mol. Struct. 4 (1969) 435.
Ab initio SCF study of the barrier to internal rotation in simple amides. Part 3. Thioamides Nikolay G. Vassilev, Valentin S. Dimitrov* Institute of Organic Chemistry, Bulgarian Academy of Sciences, Sofia 1113, Bulgaria Received 23 December 2002; accepted 10 February 2003
Abstract The free energies of activation for rotation about the thiocarbonyl C– N bond in X– C(S)N(CH3)2 (X ¼ H, F, Cl, CH3, CF3) were calculated at the MP2(fc)/6-31 þ G*//6-31G* and MP2(fc)/6-311þþ G**//6-311þ þG** levels and compared with literature NMR gas-phase data. The results of calculations indicate that the nonbonded interactions in ground state (GS) are mainly responsible for the differences in the rotational barriers. For X ¼ H, CH3 and CF3, the anti transition state (TS) is more stable; for the case X ¼ Cl, the syn TS is more stable, while for the X ¼ F, the two TS are energetically almost equivalent. q 2003 Elsevier Science B.V. All rights reserved. Keywords: Thioamides; Barrier to internal rotation; Ab initio SCF study
1. Introduction Amides are simple models for the peptide bond in proteins. The internal rotation about the amide C –N bond in amides and thioamides have been intensively studied experimentally by NMR spectroscopy in the gas [1 –7] and in the liquid phase [8]. The experimental results were used to judge theoretical methods of calculating barrier heights. The origin of the C –N rotational barrier and its relation to amide resonance have also received much attention in the last years [9 – 14]. We were interested more in the origin of the differences in the rotational barriers and in Part 1 of this series [15] we found that in case of * Corresponding author. Tel.: þ359-2-9606-189; fax: þ 359-28700-225. E-mail address: [email protected] (V.S. Dimitrov).
X –C(O)N(CH3)2 (X ¼ H, F, Cl and Br) the repulsion between X and methyl group in ground state (GS) and the repulsion between X or oxygen and nitrogen lone pair in transition states (TS) are largely responsible for the difference in the free energies of the studied amides. In Part 2 of this series [16] we conclude that in case of R – C(O)N(CH3)2 (R ¼ CH3, CH2F, CHF2, CF3 and CCl3) the repulsion between the substituent R and the methyl group in GS and the conformation of R itself are mainly responsible for the difference in the free energies of the studied amides. We have continued this study by ab initio calculations of rotational barriers of thioamides: N; N-dimethylthioformamide (DMTF), N; Ndimethylthiocarbamyl fluoride (DMTCF), N; N-dimethylthiocarbamyl chloride (DMTCCl), N; N-dimethylthioacetamide (DMTA) and N; Ndimethyltrifluorothioacetamide (DMTFTA).
0022-2860/03/$ - see front matter q 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0022-2860(03)00177-7
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2. Methods The ab initio SCF calculations were performed using the GAMESS package [17]. The complete geometry optimization was carried out using the 631G* and 6-311þ þ G** basis sets, which have been shown to provide excellent results for the structures of neutral molecules [18]. The molecules were assumed to have C1 symmetry in the GS and CS symmetry in the TSs (Fig. 1). The energies were then calculated at the MP2(fc)/6-31 þ G*//6-31G* and at the MP2(fc)/ 6-311þ þ G**//6-311þ þ G** levels of theory. The Hessians were calculated numerically with the 631G* basis set using HF/6-31G* optimized geometry. In calculating vibrational energies, the vibrational frequencies were scaled by a factor of 0.89 [18]. In each case, seven (for the TS) or eight (for the GS) scaled frequencies below 500 cm21 were treated as rotations ðE ¼ RT=2Þ [18]. The imaginary frequency for the TSs is ignored in all thermodynamics calculations.
3. Results and discussion 3.1. Geometry optimization The heavy-atom framework of the studied thioamides in the GS was found to be essentially planar and very close to CS symmetry. The calculated energies are presented in Table 1 and the selected geometry parameters appear in Table 2. The substitution of H atom in DMTF with halogen atom shortens the CyS ˚ while the same substitution bond with less then 0.01 A
˚. in oxoamides shortens the CyO bond with 0.02 A This suggests that the CyS group is less affected by electron withdrawing or electron donating substituents and in thioamides nonbonded interactions will be more important. Therefore we wish to single out the conformation of the methyl groups syn and anti-to the thiocarbonyl group (Fig. 1). In all compounds the dihedral angle C2 – N4 – C5 –H7 is about 608, while in the case of DMTF this dihedral angle is about 08. This suggests that the S· · ·H nonbonded repulsion is significant and the syn methyl groups are rotated in order not to be eclipsed with the sulfur atom in contrast to oxoamides in which the methyl hydrogens are almost eclipsed with the carbonyl oxygen [15]. Recently the MP2/6-311 þ G** ab initio calculations of DMTF [14] result in structure with dihedral angle C2 –N4 – C5 –H7 of about 608. In all compounds the dihedral angle C2 – N4 – C6 –H10 is about 08, while in the case of DMTCClF this dihedral angle is about 608. This suggests that in case of DMTCClF the repulsion between chlorine and anti methyl group is significant and the anti methyl group is rotated in order not to be eclipsed with the chlorine. Similar is the conformation of anti methyl groups in oxoamides X –C(O)N(CH3)2 (X ¼ Cl and Br), in which the repulsion interaction between halogen atoms and anti methyl group is responsible for increasing the energy of the GSs and lowering of the C – N rotational barrier [14]. The calculated dipole moment of DMTF (5.77 and 5.82 D, Table 2) is in good agreement with the previously observed value (4.74 D) [19]. The TS geometries of DMTF, DMTCF, DMTCCl, DMTA, DMTFTA (Fig. 1) were optimized in CS symmetry. The calculated energies, selected geometry
Fig. 1. Conformations and numbering of the studied thioamides (X ¼ H, F, Cl, CH3, CF3).
N.G. Vassilev, V.S. Dimitrov / Journal of Molecular Structure 654 (2003) 27–34
29
Table 1 Calculated energies for the studied thioamides X –C(S)NR2 in Hartrees X
R
State
ZPEa
HF/6-31G*
MP2(fc)/6-31 þ G*//6-31G*
HF/6-311þ þG**
MP2(fc)/6-311þ þG**// 6-311þ þ G**
H H H F F F Cl Cl Cl CH3 CH3 CH3 CF3 CF3 CF3 H H H F F F Cl Cl Cl CH3 CH3 CH3 CF3 CF3 CF3
CH3 CH3 CH3 CH3 CH3 CH3 CH3 CH3 CH3 CH3 CH3 CH3 CH3 CH3 CH3 H H H H H H H H H H H H H H H
GS anti TS syn TS GS anti TS syn TS GS anti TS syn TS GS anti TS syn TS GS anti TS syn TS GS anti TS syn TS GS anti TS syn TS GS anti TS syn TS GS anti TS syn TS GS anti TS syn TS
60.32 59.53 59.63 55.81 55.40 55.32 54.88 54.33 54.26 77.19 76.45 76.29 63.93 63.28 63.13 26.50 25.74 25.53 21.99 21.46 21.26 21.00 20.42 20.33 43.17 42.54 42.36 30.14 29.37 29.26
2569.626326 2569.588497b 2569.587683 2668.483936 2668.451205 2668.451901b 21028.516442 21028.489492 21028.490245b 2608.660989 2608.632606b 2608.626562 2905.228680 2905.204307b 2905.197218 2491.566677 2491.532598b 2491.528849 2590.425864 2590.394101b 2590.393320 2950.463266 2950.431922b 2950.431794 2530.609163 2530.577775b 2530.571258 2827.177960 2827.147050b 2827.144978
2570.310144 2570.276525b 2570.275910 2669.345169 2669.318980b 2669.318863 21029.341586 21029.317808 21029.319192b 2609.481063 2609.456369b 2609.449920 2906.578808 2906.554055b 2906.546301 2491.984423 2491.953768b 2491.951126 2591.021058 2590.994454 2590.995099b 2951.018256 2950.990606 2950.991442b 2531.159355 2531.132445b 2531.127274 2828.257055 2828.228660b 2828.227499
2569.697307 2569.660418b 2569.659430 2668.584573 2668.553968b 2668.553616 21028.615270 21028.588901 21028.589854b 2608.741696 2608.714040b 2608.708098 2905.394909 2905.371524b 2905.363904 2491.624113 2491.591121b 2491.588137 2590.513396 2590.483482b 2590.482418 2950.548601 2950.518016 2950.518217b 2530.676304 2530.645941b 2530.640017 2827.330172 2827.301222b 2827.299363
2570.458973 2570.426802b 2570.426201 2669.549753 2669.524987b 2669.524122 21029.521700 21029.498513 21029.500024b 2609.660446 2609.636037b 2609.629925 2906.924248 2906.900232b 2906.891974 2492.078396 2492.049365b 2492.047338 2591.171236 2591.146327 2591.146613b 2951.142821 2951.116584 2951.117674b 2531.283022 2531.257572b 2531.253009 2828.547623 2828.520395b 2828.519227
a b
ZPE is reported in kcal/mol at the HF/6-31G* level scaled by 0.89. More stable TS.
parameters for anti and syn TSs, are presented in Tables 1, 3 and 4, respectively. The anti TS is more stable in the cases of DMTF, DMTA and DMTFTA, in the case of DMTCCl the synTS is more stable, while in the case of DMTCF the two TS are energetically almost equivalent. The most significant structural changes in the process of rotation towards the TSs are that the nitrogen is pyramidalized, the C – N bond lengthens ˚ and the CyS bond from 1.32– 1.33 to 1.39 –1.43 A ˚ . However, shortens from 1.65 –1.67 to 1.60 –1.62 A the CyO bond length in corresponding amides ˚ only. This indicates that shortens by 0.01– 0.02 A the thiocarbonyl group is relatively more affected by
this rotation in comparison to carbonyl group in the oxoamides. 3.2. Comparison of calculated activation parameters and experimental data Calculation of the vibrational frequencies confirmed the assignment of the anti and syn forms as TSs and allowed computation of the enthalpy, entropy and free energy changes at 298 K. The thermodynamic results for the isomerisation of the studied amides are presented in Table 5. DH – (298 K) is obtained from the sum of the changes in the electronic energy, DEeo ; the zero-point vibrational energy, DEvo ; and
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N.G. Vassilev, V.S. Dimitrov / Journal of Molecular Structure 654 (2003) 27–34
Table 2 Selected structural parameters and dipole moments (m) for GS of the studied thioamides Parameters Bond lengths S1 –C2 C2 –X3 C2 –N4 N4–C5 N4–C6 S1 –H7 Bond angles S1 –C2–N4 X3–C2 –N4 C2 –N4–C5 C2 –N4–C6 Dihedral angles C2 –N4–C5– H7 C2 –N4–C6– H10 S1 –C2–N4– C5 X3–C2 –N4–C6 m
X¼H
1.653(1.654) 1.078(1.080) 1.321(1.319) 1.452(1.453) 1.451(1.452) 2.649(2.644)
X¼F
1.648(1.646) 1.314(1.312) 1.317(1.316) 1.459(1.460) 1.459(1.461) 2.996(2.996)
X ¼ Cl
1.648(1.647) 1.752(1.756) 1.324(1.322) 1.465(1.466) 1.465(1.466) 2.910(2.917)
X ¼ CH3
1.670(1.670) 1.515(1.515) 1.329(1.328) 1.459(1.459) 1.455(1.456) 2.902(2.904)
X ¼ CF3
1.651(1.649) 1.539(1.543) 1.324(1.323) 1.461(1.642) 1.464(1.645) 2.890(2.894)
129.5(129.5) 112.2(112.6) 123.4(123.3) 121.4(121.2)
128.5(128.6) 112.1(112.2) 119.5(119.4) 123.6(123.8)
126.8(127.0) 114.2(114.2) 117.9(118.0) 123.5(123.6)
123.0(123.0) 117.8(117.9) 120.3(120.0) 124.9(124.8)
125.1(125.2) 116.2(116.3) 118.6(118.6) 127.4(127.4)
0.0(0.0) 0.0(0.0) 0.0(0.0) 0.0(0.0) 5.77(5.82)
60.2(60.2) 0.0(0.0) 0.0(0.0) 0.0(0.0) 5.73(5.73)
59.9(59.9) 60.8(60.8) 0.0(0.0) 0.0(0.0) 5.52(5.51)
60.1(60.1) 20.3(20.3) 0.0(0.0) 0.0(0.0) 5.76(5.79)
59.5(59.9) 0.2(0.2) 0.0(0.0) 0.0(0.00) 5.28(5.35)
The parameters are given for HF/6-31G* optimized geometry while the values in parenthesis correspond to HF/6-311þ þG** optimized ˚ and bond angles are given in deg. Dipole moments are given in Debye. For numbering of atoms see Fig. 1. geometry. Bond lengths are given in A
the thermal correction to the zero-point energy, DDEv298 : The scaled frequencies were used also for the entropy calculation. The two lowest real frequencies, which correspond to rotations of the methyl groups,
need to be treated as hindered rotations. The resultant free energy DG– (298 K) for the anti TS (DMTF, DMTA and DMTFTA) and for the syn TS (DMTCCl) should be compared with the gas-phase NMR results.
Table 3 Selected structural parameters and dipole moments ðmÞ for anti TS of the studied thioamides Parameters Bond lengths S1 –C2 C2 –X3 C2 –N4 N4–C5(N4–C6) Bond angles S1 –C2–X3 S1 –C2–N4 C2 –N4–C5 (C2– N4– C6) Dihedral angles S1 –C2–N4– C5 X3–C2 –N4–C6 m
X¼H
1.608(1.607) 1.080(1.082) 1.419(1.419) 1.458(1.459)
X¼F
1.609(1.606) 1.307(1.306) 1.395(1.393) 1.461(1.462)
X ¼ Cl
1.605(1.604) 1.743(1.748) 1.403(1.402) 1.459(1.460)
X ¼ CH3
1.619(1.619) 1.500(1.499) 1.429(1.429) 1.457(1.458)
X ¼ CF3
1.599(1.598) 1.527(1.532) 1.414(1.413) 1.459(1.460)
119.2(119.1) 127.6(127.5) 113.0(113.0)
120.2(120.0) 129.1(129.2) 113.4(113.3)
121.0(120.7) 127.3(127.4) 113.3(113.2)
123.6(123.4) 124.5(124.3) 113.0(113.0)
122.5(122.1) 127.4(127.5) 113.4(113.3)
264.6(264.7) 2115.4(2115.3) 2.10(2.10)
265.4(265.4) 2114.6(2114.6) 3.06(3.04)
265.7(265.6) 2114.3(2114.4) 2.69(2.69)
264.4(264.4) 2115.6(2115.6) 2.47(2.44)
265.5(265.3 2114.5(2114.7) 2.72(2.96)
The parameters are given for HF/6-31G* optimized geometry while the values in parenthesis correspond to HF/6-311þ þG** ˚ and bond angles are given in deg. Dipole moments are given in Debye. For numbering of optimized geometry. Bond lengths are given in A atoms see Fig. 1.
N.G. Vassilev, V.S. Dimitrov / Journal of Molecular Structure 654 (2003) 27–34
31
Table 4 Selected structural parameters and dipole moments (m) for syn TS of the studied thioamides Parameters Bond lengths S1 –C2 C2 –X3 C2 –N4 N4–C5(N4–C6) Bond angles S1 –C2–X3 S1 –C2–N4 C2 –N4–C5 (C2–N4–C6) Dihedral angles S1 –C2–N4–C5 X3–C2 –N4–C6 m
X¼H
1.600(1.599) 1.087(1.089) 1.413(1.412) 1.457(1.458)
X¼F
1.600(1.598) 1.323(1.332) 1.390(1.389) 1.462(1.463)
X ¼ Cl
1.597(1.596) 1.774(1.778) 1.393(1.393) 1.458(1.459)
X ¼ CH3
1.613(1.613) 1.511(1.510) 1.426(1.426) 1.453(1.454)
X ¼ CF3
1.598(1.596) 1.535(1.540) 1.408(1.408) 1.455(1.457)
119.5(119.2) 125.5(125.7) 110.6(110.5)
120.2(119.9) 127.7(127.7) 112.5(112.7)
120.0(119.8) 125.4(125.7) 114.3(114.1)
122.3(122.2) 120.6(120.5) 114.1(113.8)
120.7(120.3) 123.3(123.4) 117.2(117.2)
2117.5(2117.6) 262.5(262.4) 3.42(3.41)
2115.3(2115.0) 264.7(265.0) 3.38(3.30)
2113.0(2113.2) 267.0(266.8) 3.00(2.98)
2113.7(2114.0) 266.3(266.0) 3.80(3.81)
2108.6(2108.6) 271.4(271.4) 2.39(2.52)
The parameters are given for HF/6-31G* optimized geometry while the values in parenthesis correspond to HF/6-311þ þG** ˚ and bond angles are given in deg. Dipole moments are given in Debye. For numbering of optimized geometry. Bond lengths are given in A atoms see Fig. 1.
In the case of DMTCF the two TS have similar energies and the resultant effective free energy DG– eff (298 K) takes into account the two possible routes and it is calculated by summing the rates through the two possible TS. It is seen from Table 5 that the barriers were calculated very satisfactorily. More precise results for the rotational barrier in amides and thioamides can be obtained using some of the composite methods like G2MP2 [16]. 3.3. Effect of the substituents on the rotational barrier It is worth to note that the experimental rotational barriers of the studied compounds X – C(S)N(CH3)2 in the gas phase follow the trend: H . F . CH3 . CF3 . Cl and correlate mainly with the substituent size and not with the substituent electronegativity. In order to find the reason for this tendency we examined the following reaction both in the GS and in the two possible TSs: H – C(S) – N(CH3)2 þ X – C(S) – NH2 ! H – C(S) – NH2 þ X – C(S) –N(CH3)2 The calculated enthalpy of the reaction for the GS, DDH 0 (GS), will be an estimate for the difference in repulsion between X and the CH3 group. The calculated DDH 0 (GS) values are the highest values
in Table 6 and therefore the repulsion between X and the anti CH3 group is mainly responsible for the differences in the rotational barriers in thioamides. The computed enthalpy of the reaction for the anti TS DDH 0 (anti TS) will be an estimate for the difference in repulsion between X and the thioamide lone pair, while the calculated enthalpy of the reaction for syn TS DDH 0 (syn TS) will be an assessment for the difference in repulsion between sulphur and thioamide lone pair. The DDH 0 (anti TS) values are very small and close to zero which means that there is no significant difference in the repulsion between X and amide lone pair in all studied compounds. The DDH 0 (syn TS) values are very small and close to zero in the case of X ¼ F and Cl, which indicate that the repulsion between X and amide lone pair is not important for restricted rotation in these compounds (DMTCF and DMTCCl). The combined energy differences for both GS and anti TS [DDH 0 (anti TS) 2 DDH 0 (GS)] should be compared with the calculated differences in the computed enthalpy of activation DDH – (0 K) for the anti TS, while the combined energy differences for both GS and syn TS ½DDH 0 (syn TS)2DDH 0 (GS)] should be compared with the calculated differences in the calculated enthalpy of activation DDH – (0 K) for the syn TS. [DDH 0 (anti TS)2DDH 0 (GS)] and [DDH 0 (syn TS)2DDH 0 (GS)] can also be compared with
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N.G. Vassilev, V.S. Dimitrov / Journal of Molecular Structure 654 (2003) 27–34
Table 5 Calculated barriers of the studied thioamides X –C(S)N(CH3)2 in the gas phase X
Methoda
TS
DH – (0 K)
DH – (298 K)
DS– (298 K)
DG– (298 K)
DG– eff (298 K)
H
1
anti syn anti syn anti syn anti syn
20.3 20.8 22.4 23.1 20.2 20.6 21.3 21.2
21.3
16.0 16.0 18.8 18.9 15.1 15.6
anti syn anti syn anti syn anti syn
14.8 18.6 16.6 20.2 14.6 18.2 16.9 19.5
anti syn anti syn anti syn
18.6 20.1 20.0 21.8 17.6 18.7
anti syn anti syn anti syn
14.4 13.4 16.0 15.3 14.0 13.0
28.9 28.7 28.9 28.7 28.9 28.7 25.3 25.6 20.7 ^ 2.5 27.0 26.8 27.0 26.8 27.0 26.8 23.0 ^ 3.0 26.4 25.1 26.4 25.1 26.4 25.1 25.6 24.8 23.3 ^ 2.0 26.3 24.7 26.3 24.7 26.3 24.7 20.1 ^ 3.0 25.8 25.6 25.8 25.6 25.8 25.6 20.2 ^ 4.0
21.5 22.1 23.6 24.4 22.0 22.5 22.6 22.5
anti syn anti syn anti syn
18.9 19.5 20.9 21.8 19.4 19.9 20.7 20.6 22.3 ^ 1.5 14.9 14.9 17.7 17.8 14.0 14.4 17.2 ^ 1.1 13.4 17.2 15.2 18.8 13.2 16.8 15.4 18.9 17.1 ^ 1.0 17.4 18.8 18.8 20.5 16.4 17.4 17.2 ^ 1.1 13.2 12.2 14.8 14.1 12.8 11.8 16.8 ^ 1.3
2 3 4 Exptlb 1
F
2 3
CH3
Exptlb 1 2 3 4
CF3
Exptlb 1 2 3
Cl
Exptlb 1 2 3 Exptlb
a b
17.0 16.9 19.8 19.8 16.1 16.5 15.4 19.3 17.3 20.8 15.3 18.9 17.4 20.5 19.5 20.9 20.9 22.6 18.4 19.5 15.3 14.3 16.9 16.2 14.9 13.8
23.4 21.8 21.9 22.5 ^ 0.1 16.5 19.4 15.8 18.3 ^ 0.1 15.4 17.3 15.3 17.3 18.0 ^ 0.1 19.4 20.9 18.4 17.2 ^ 0.1 14.2 16.0 13.7 16.9 ^ 0.1
DG– eff (298 K) is calculated by summing up the rates through the two possible TS. (1) MP2(fc)/6-31 þ G*//6-31G*, (2) HF/6-311þ þ G**, (3) MP2(fc)/6-311þþ G**//6-311þþ G**, (4) G2MP2 at 353 K from Ref. [12]. Ref. [7].
the calculated change in the free energy DDG– eff (298 K) and with the experimental change in the free energy of activation DDG– exp (298 K). Since the anti TS is the preferred one in the cases of DMTA and DMTFTA, the values in the row [DDH 0 (anti TS) 2 DDH 0 (GS)] should be compared with energy – differences DDG– eff (298 K) and DDGexp (298 K). It is
seen from Table 6 that the calculated [DDH 0 (anti TS) 2 DDH 0 (GS)] contributions are of the order of magnitude of the experimental energy differences DDG– exp (298 K). In the case of DMTCCl the synTS is more stable and therefore the values in the row [DDH 0 (syn TS) 2 DDH 0 (GS)] should be compared with energy differences DDG– and eff (298 K)
N.G. Vassilev, V.S. Dimitrov / Journal of Molecular Structure 654 (2003) 27–34
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Table 6 Origin of the difference in the rotational barriers (kcal/mol) of the studied thioamides X– C(S)N(CH3)2 in the gas phase. H – C(S)– N(CH3)2 þ X– C(S)–NH2 ! H –C(S)–NH2 þ X –C(S)–N(CH3)2 X
H
F
DDH 0 (GS) DDH 0 (anti TS) DDH 0 (syn TS) anti TS –GS: DDH – (0 K) DDH 0 (anti TS)–DDH 0 (GS) syn TS –GS: DDH – (0 K) DDH 0 (syn TS)–DDH 0 (GS) DDG– eff (298 K) DDG– exp (298 K)
0(0) 0(0) 0(0)
1.0(1.3) 20.6(20.6) 0.1(0.0)
0(0) 0(0) 0(0) 0(0) 0(0) 0
CH3
CF3
Cl
5.1(5.1) 0.8(0.9) 2.0(1.8)
5.6(5.3) 20.6(20.4) 3.9(4.0)
4.1(4.1) 20.9(20.9) 0.1(20.4)
24.3(23.6) 21.6(21.9)
25.5(25.8) 24.3(24.2)
21.7(22.4) 26.2(25.7)
25.9(26.4) 25.0(25.1)
24.8(24.1) 20.9(21.3) 24.8(24.0) 24.2
22.2(22.9) 23.1(23.3) 25.9(26.1) 24.5
20.7(21.3) 21.7(21.3) 21.9(22.5) 25.3
27.4(27.8) 24.0(24.6) 27.1(27.4) 25.6
DDH 0 (GS), DDH 0 (anti TS) and DDH 0 (syn TS) are the energy changes (HF/6-31G* energy with ZPE correction) for the model reaction in the ground, anti transition and syn TSs, respectively. DDH – (0 K) is the difference in the calculated enthalpy of activation for the amide rotation, – DDG– eff (298 K) is the change in the calculated free energy and DDGexp (298 K) is the experimental change in free energy. The values in parenthesis correspond to the HF/6-311þ þG** level of theory.
DDG– exp (298 K). Again the calculated energy contributions [DDH 0 (syn TS) 2 DDH 0 (GS)] are similar to the experimental energy differences DDG– exp (298 K). In the case of DMTCF the two TS are energetically almost equivalent and the values in both rows [DDH 0 (anti TS) 2 DDH 0 (GS)] and [DDH 0 (syn TS) 2 DDH o (GS)] should be compared with the two energy differences DDG– and eff (298 K) – DDGexp (298 K). As it is seen from Table 6 these contributions to the experimental energy difference DDG– exp (298 K) depend on the level of calculation and are between 20% and 45%.
4. Conclusion The results of our calculations indicate that the nonbonded interactions (mainly the repulsion between X and anti CH3 in GS) are responsible for the differences in the free energies of rotation of the studied thioamides. This conclusion is based on the following facts: 1. The CyS group is less affected by electron withdrawing or electron donating substituents, as compared to the CyO group in oxoamides. 2. The S· · ·H nonbonded repulsion is significant and the syn methyl groups are rotated in order not to be eclipsed with the sulfur atom in comparison to
oxoamides in which the methyl hydrogen is almost eclipsed with the carbonyl oxygen. 3. The calculated DDH 0 (GS) values in the model reaction are the highest values in Table 6 and therefore the repulsion between X and the anti CH3 group is mainly responsible for the differences in the rotational barriers in thioamides.
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