J . Phys. Chem. 1995, 99, 556-562
556
Protonation Sites and Rotational Barriers Calculation for Formamide and Thioformamide Ming-Chiu Ou and San-Yan Chu* Department of Chemistry, National Tsing Hua University, Hsinchu 30043, Taiwan, Republic of China Received: May 11, 1994; In Final Form: October 13, 1994@
The N- and 0-site protonation affinities for formamide and thioformamide were studies by ab initio molecular orbital calculations using the 6-31G** basis set at the Hartree-Fock and up to the MP4 level. The proton affinity for 0-site is found to be more favorable than that for the N-site by 13-26 kcal mol-', dependent on the level of calculation. The natural bond orbital analysis shows that the 0-site protonation can enhance the zwitterionic contribution and thus increase the rotational barrier around the C-N bond. The N-site protonation has the opposite effect. For an 0-site protonation, both 0-and n-orientation were considered. We also investigated a weaker interaction of the hydrogen bond at the two sites with the HF molecule as a hydrogen donor. We made a comparison between formamide and thioformamide.
Introduction The amino group is known to be a stronger base than the carbonyl group. For example, the proton affinity of ammonia is 204 kcal mol-' compared with the value 172 kcal mol-' for formaldehyde.' However, the protonation site in most amide ~ be systems is on the 0 atom rather than the N a t ~ m . ~It .can rationalized that the resonance structure (zwitterionic structure) can simultaneously increase the basicity of 0 and reduce the basicity of N due to the delocalization of the N lone pair to form a partial C-N x-bond, as shown in la. In fact, the
,r
-0
\
H/ ' = N \
H la
rotational barriers in amides are about 15-20 kcal mol-', much bigger than that of a typical single bond. For the perpendicular conformation with the amine group rotated 90" from the planar conformation in which the resonance structure is no longer feasible, one may predict the more preferable protonation site is again on the N-site. One interesting consequence of this torsional angle dependence of proton affinity is that there exists certain intermediate conformations with the torsional angle near 45" where 0 and N may have equal proton affinities. Such interesting systems do exist. Greenberg and Venanzi3recently discussed a class of strained amide systems with reduced resonance contributions to different extents from nonplanar distortion and the consequences of N- and 0-protonation. In this work, we calculated the proton affinities and the hydrogen-bonding strengths of HF associated with either the 0-site or N-site in formamide for the planar conformation as well as the perpendicular conformation. From these results, we can obtain information about the protonation and the hydrogenbonding effects on the C-N rotational barrier. A special interest is that the interaction of an acid at the 0-site stabilize the local negative charge in the resonance structure and thus increase the rotational barrier. A comparison between the protonation effect and the hydrogen-bonding effect of HF is also of interest in view that the former interaction is strong enough to create a genuine C-N double bond. For a much weaker interaction in @Abstractpublished in Advance ACS Abstracts, December 1, 1994.
the latter, we may examine the importance of resonance structure by the second-order perturbation energy E(2) for the occupied lone pair of N delocalized to the vacant n*co. This quantity can be compared with that for the free amide system. We will also study the thioformamide system to see the second-rowelement effect with S-substitution for 0.4 We note there are two possible orientations for protonation and hydrogen bonding on the 0 atom: one is an in-plane orientation (denoted as 0,) and the other is out-of-plane (denoted as Ox).The overall planning of this study is that the formamide (FA) and the thioformamide (TFA), each in planar and perpendicular conformation, represent the four variations in the importance of the resonance structures shown in l a . Each is modified six ways by an acid, either a proton or HF, interacting at the three sites (N, 0,, Ox).From these total 24 systems plus the four systems for the free molecules, we have the chance to see the tuning of the importance of the resonance structure and the corresponding rotational barrier in the various acid environments. Hopefully, some rules can be established for rationalization of the relative magnitude of these barriers. Theoretical Method
Ab initio LCAO-MO SCF and frozen core Moller-Plesset perturbation (MP2, MP4SDTQ) calculations were performed with the 6-31G** basis set using the GAUSSIAN 92 p r ~ g r a m . ~ Jasien et aL6 emphasized that the polarization functions on N are important for reliable representation of the out-of-plane deformations and accurate determination of the NH2 rotational barrier. For N-site protonation, there are two possible conformers, Le. with the in-plane N-H bond syn and anti, respectively, to the C - 0 group (see Scheme 1). The syn conformer is 0.64 kcal mol-' more stable than the anti conformer for FA and 1.25 kcal mol-' for TFA. For O(S)&e protonation and hydrogen bonding, we only considered the more stable anti conformer (see Scheme 1 for definition). The energy difference between the two 0-protonated isomers is 3.26 kcal mol-' for FA and 0.77 kcal mol-' for TFA. For the O(S),-site interaction, the proton and HF were assumed to approach the molecule in the vertical plane. Only the linear configurations for O(S). .OH-F and N-v-H-F were considered. The structures of all the rotamers are displayed in Schemes 1 and 2. The geometry was fully optimized for the HF and MP2 levels by default in the Gaussian92 package, and the harmonic vibrational frequencies were calculated using normal vibration mode analysis. For the
0022-3654/95/2099-0556$09.00/0 0 1995 American Chemical Society
Protonation Sites for Formamide and Thioformamide
SCHEME 1
J. Phys. Chem., Vol. 99,No. 2, 1995 557 TABLE 1: Calculated Geometrical Parameters for FA and "FA with Units of Angstroms for Bond Lengths and Degrees for Bond Angles (See 1 in Scheme 1)
X
~~
free
/
C#-N
H
\HZ
___)
/c-N
planar
H
1
2
CN
co
CH NHI NHZ
N-site
3
4
X
co
H
?&-site H ' \ & l (anti)
LOCN LHCN LCNHi LCNHz nonplanar CN CH NHI NHz LOCN LHCN LCNHl LCNHz LOCNH LOCNHi LOCNH2
&xu,
I
\H
H
5
H
/c-N 6
planar
CN
cs
CH NHI
NH2 LSCN LHCN LCNHl LCNH;?
X = 0 (FA)or S (TFA)
SCHEME 2 a
9
11
10
..
12
X = 0 (FA)or S (TFA)
MP4 level, the geometry optimization used was the FletcherPowell numerical method. The optimized structures for all the rotamers were confmed by the number of imaginary frequencies. We found that the population analysis for the occupancies of the natural bond orbitals (NBO)' and the second-order donor-acceptor perturbation energy E(2)8 are useful for interpreting the relative importance of the resonance structure and the rotational barriers for these systems. Geometry The optimized geometries for the ground states FA and TFA at various theoretical levels are summarized in Table 1. The gas phase electron diffraction (ED)9910data are available for FA
Microwave
HF MP2 MP4 Formamide (FA) 1.3478 1.3614 1.3652 1.1931 1.2240 1.2280 1.0924 1.1011 1.1040 0.9940 1.0055 1.0066 0.9912 1.0032 1.0041 124.862 124.773 124.767 112.794 112.116 112.045 119.023 118.853 118.921 121.632 121.637 121.719 1.3479 1.3619 1.3661 1.1930 1.2241 1.2281 1.0924 1.1011 1.1040 0.9939 1.0057 1.0065 0.9912 1.0034 1.0044 124.901 124.773 124.737 112.772 112.170 112.085 119.051 118.488 118.571 121.623 121.333 121.407 180.020 180.947 180.940 0.023 5.038 5.380 180.061 173.689 173.181 Thioformamide ( F A ) 1.3234 1.3472 1.3518 1.6407 1.6353 1.6464 1.0804 1.0887 1.0907 0.9951 1.0075 1.0079 0.9931 1.0051 1.0059 126.393 126.156 126.178 113.096 112.281 112.186 119.613 119.316 119.465 121.532 121.622 121.476
experimental 1.352" 1.224 1.098 1.003 1.001 124.5 114.6 118.4 119.6
1.368* 1.212 1.125 1.027 1.027 125.0 112.7 118.7 119.0
1.358" 1.626 1.096 1.002 1.007 125.16 108.05 117.55 120.22
Electron diffraction
only. However, there are microwave (MW)11J2data available for both FA and TFA. Since it is still experimentallyunresolved whether FA is planar,13 this calculation explored both C1 and C, symmetry for FA. There is no significant difference between optimized C1 and C, geometries at the Hartree-Fock level; however, the difference becomes more important at the MP2 and MP4 levels (Table 1). Their geometry parameters are in reasonable agreement with the experimental values. For TFA, the electron correlation is more important for the heavy atom sulfur, which results in an important difference for C=S bond length between MP2 and MP4 calculations. For both systems, the MP4 results are generally closer to the experimental values than the HF and MP2 results. Therefore, we are interested in some comparison between FA, TFA, and their rotamer results at the MP4 level. Most of our discussion in this paper is referred to the MP4 result if not specified otherwise. For various rotamers, 2-14, the CN and CO bond length differences with respect to those of 1 shown in Table 2, we can observe some C-O(S) bond shortening and C-N lengthening in 2 in comparison with those in 1; therefore, we expect that the zwitterionic structure already makes some important contribution for the free molecules. For the perpendicular conformer of the free molecules and N-protonated molecules, the lone pair of N is no longer available for delocalization and thus the zwitterionic contribution diminishes; therefore the C-O(S) and C-N bonds assume their normal bond lengths. For the perpendicular conformer of FA (2), the change for RCN is +0.0849 8, and that for Rco is -0.0074 A. The corresponding values for TFA are +0.0928 and -0.0151 A. For N-protonation (3), the corresponding values are +0.2116 and -0.0372 A for FA and +0.1743 and -0.0472 8, for TFA with the magnitude
Ou and Chu
558 J. Phys. Chem., Vol. 99, No. 2, 1995
TABLE 2: Calculated Bond Lengths (A) of CN and CO(S) Bonds for Free (2), Protonated, and HF-Bonded FA and TFA (Bond Lengths Are Expressed as the Differences between the Rotamer and Free Molecule 1 As Displayed in Table 1" protonated species free 2
3
4
0.0770 0.0811 0.0849 -0.0100 -0.0071 -0.0074
0.1742 0.2039 0.2116 -0.0358 -0.0366 -0.0372
0.0973 0.0898 0.0928 -0.0358 -0.0154 -0.0151
0.1673 0.1685 0.1743 -0.0718
5
6
7
8
-0.0690 -0.0656 -0.0660 0.0799 0.0723 0.0732
0.0107 0.0007 0.0038 0.0518 0.0482 0.0498
-0.0754 -0.0704 -0.0701 0.1004 0.0881 0.0887
0.0132 0.0008 0.0010 0.0562 0.0298 0.0349
-0.0420 -0.0432 -0.0430 0.0646 0.0589 0.0587
0.0529 0.0362 0.0394 0.0035 0.0110 0.0136
-0.0567 -0.0592 -0.0587 0.1350 0.1285 0.1278
0.0351 0.0162 0.0193 0.1303 0.1188 0.1177
Formamide (FA) M C N
HF
MP2 MP2 Mco
HF
MP2 MP4
0.1815 0.2284 0.2242 -0.0379 -0.0379 -0.0393 Thioformamide ( F A )
M C N
HF
MP2 MP4 ARCS
HF MP2 MP4
0.1674 0.1797 0.1857 -0.07 15 -0.0455 -0.0482
-0.0444
-0.0472
HF-bonded species
9
10
11
12
13
14
0.0686 0.0689 0.0742 -0.0027 0.0014 0.0007
-0.0108 -0.0128 -0.0128 0.0113 0.0116 0.0115
0.0667 0.0647 0.0716 -0.0027 0.0007 0.0011
0.0928 0.0842 -0.0128 -0.0297 -0.01 11 -0.0023
-0.0074 -0.0079 -0.0075 0.0116 0.0084 0.0079
0.0868 0.0734 0.075 -0.0302
Formamide (FA) M C N
HF
ARC0
MP2 MP4 HF MP2 MP4
M C N
HF
0.0295 0.0322 0.0740 -0.0074 -0.0068 -0.0072
0.0817 0.0963 -0.0166 -0.0143 -0.01 16
0.0252 0.0324 0.0352 -0.0158 -0.0123 -0.0129
0.1034 0.0969 0.1023 -0.0472 -0.0248 -0.0200
0.0855
-0.01 17 -0.0143 -0.0144 0.0093 0.0112 0.0112
Thioformamide ( P A )
MP2 MP4 ARCS
HF
MP2 MP4 a
-0.0068 -0.0081 -0.0078 0.0104 0.0089 0.0093
-0.0105
-0.0096
See Schemes 1 and 2 for numbering for each rotamer.
of ARCNmuch larger than ARco(s). The O(S)-site protonation (5, 7)results in full CN double-bond character, as depicted in Scheme 1, which will be discussed in more detail later. For Orsite protonation (9,the change in the C-0 bond is +0.0732 A and that in the C-N bond is -0.0660 A for FA, and the corresponding values for the &site in TFA are +0.0587 and -0.0430 A. For O(S),-site protonation (7),the corresponding quantities are +0.0887 and -0.0701 8, in FA and +0.1278 and -0.0587 8, in TFA. The result is in accord with the exception that protonation on the oxygen site can enhance the C=N structure effectively. It is interesting to note that protonation at the O(S)rsite has greater effects on bond lengths than that at the O(S),-site. It can be rationalized easily since the resonance structure is referred to the delocalization effect of the x-electrons. Therefore, the O(S),-protonation affects the resonance structure more directly. However, the O(S),-protonation is less energetically favorable than the O(S),-protonation by 11.43(11.97) kcal mol-'. This difference can be understood easily in terms of the isoelectronic species C3H5-. The allylic species is known to be 19.0 kcal mol-' more stable than the vinylic species.14 For 0,-protonation in FA, the change in the CO bond length, is slightly more than for the 0, case. However, for S,-protonation in TFA, the changes are far more important than the S, case with the ARCSvalue being twice as big. The different behaviors in TFA relative to FA will be discussed further later. Table 3 shows the local geometries for the protonated and HF-bonded species with X--*Hbond lengths and LCXH bond angles (X = N or O(S)) involving the interaction site. The Hartree-Fock results are significantly different from MP2 and MP4 results. For N-site-protonated and HF-bonded species (3, 4, 9, lo), the LCXH are similar between FA and TFA.
Nevertheless, there are some important differences in the bond angle between 0-site-, and S-site-protonated species (5-8, 11, 12). For 0-site protonation, the LCOH has the values 113160'. For S-site protonation, the LCSH decreases to 86-98'. In the O(S),-site HF-bonded systems (13 and 14), the angle is fixed at 90".
Proton Affinities According to the experimental procedure, the proton affinity (PA) is often defined as the negative value of the standard free H+ BH+ reaction. In theoretical energy for the B calculations, the absolute PA is defined as -AH, which can be estimated by considering the zero-point energy (ZPE), vibration energy (E& and total electronic energy Eoelecaccording to eq 1 and 2.l59l6
+
-
AE = E(products) - E(reactants)
+
+ ALEvib(r ) - (3/2)RT (1) PA(B) = -AH= -AEaelec- AZPE - AEvib(7) + (5/2)RT = AEoelec AZPE
(2) In this work, the PA value is site dependent. We would like to compare the difference between the O(S)-site and N-site for FA and TFA at ambient temperature (298 K). Only the total electronic energies Eoelecand the absolute proton affinities (PA) are listed in Table 4. To give an idea about the relative importance of AEoelec,AZPE, hEvib, and the (5/2)RT contribution to PA, the Orsite has a PA value of 197.79 kcal mol-' and these four components are -215.40,9.66,9.46, and 4.97 kcal mol-', respectively. The experimental PA of FA
Protonation Sites for Formamide and Thioformamide TABLE 3: Calculated X-*-H Bond Lengths (A) and LCX..-H (X = N or O(S))(deg) for Protonated and KE-Bonded FA and TFA protonated species 3" 4 5 6 7 Formamide (FA) RN(OF..H HF 1.0145 1.0120 0.9512 0.9584 0.9531 MP2 1.0268 1.0247 0.9725 0.9832 0.9713 MP4 1.0271 1.0268 0.9713 0.9823 0.9701 LCN(O)...H HF 108.5 113.9 115.5 116.7 118.3 MP2 108.0 113.8 113.1 114.0 117.0 MP4 108.0 113.7 112.9 113.9 116.6 Thioformamide (TFA) RN(s~..H HF 1.0130 1.0142 1.3261 1.3309 1.3315 MP2 1.0269 1.0255 1.3322 1.3398 1.3371 MP4 1.0272 1.0258 1.3353 1.3432 1.3398 LCN(S).*.H HF 110.6 111.1 95.9 98.3 93.1 MP2 109.7 112.5 94.5 97.3 91.7 MP4 109.7 112.4 94.3 97.1 91.7 HF-bonded species 11 12 13 Formamide (FA) 1.9348 1.8829 1.7672 1.8500 1.9789 1.8235 1.7737 1.6900 1.8009 1.8791 1.8393 1.7907 1.6930 1.8219 1.8831 111.5 114.7 120.9 106.0 90.P 110.8 111.7 106.3 100.4 90.0 110.8 112.3 106.6 100.4 90.0 Thioformamide (TFA) 1.9289 1.8775 2.4318 2.5246 2.5587 1.7950 1.7618 2.2796 2.3693 2.4117 1.7950 1.7765 2.2920 2.2789 2.4239 116.6 113.6 81.8 83.5 90.P 113.9 110.6 80.0 81.3 90.0 113.9 110.6 79.6 80.0 90.0 9
HF MP2 MP4 LCN(O).*.H HF MP2 MP4
RN(o~..H
RN(s~..H
HF
MP2 MP4 LCN(S).**H HF MP2 MP4
10
J. Phys. Chem., Vol. 99, No. 2, 1995 559
8
0.9552 0.9713 0.9705 138.7 159.5 153.0 1.3391 1.3503 1.3525 90.2 86.1 86.6
14 1.1075 2.0078 2.0118 90.P 90.0 90.0 2.8457 2.6502 2.6597 90.P 90.0 90.0
See Schemes 1 and 2 for numbering for each rotamer. LCXH is fixed at 90". is 198.4 kcal m01-'.'~3~*The calculated 0-protonated PA values at the HF, MP2, and MP4 levels are 197.8, 194.1, and 195.3 kcal mol-', respectively. The PA of ammonia is known to be larger than that of formaldehyde. However, the 0-protonated FA is more stable than the N-protonated one by 14 kcal mol-' e~perimentally.'~Our calculated difference is 12.44 kcal mol-'. We are interested in the protonation effect on the contribution of the resonance structure. The extent of the delocalization and the relationship to resonance structure can be analyzed in terms of the orbital occupancy and the second-order donor-acceptor perturbation E(2) from the NBO method. Some relevant orbital occupancies and E(2) stabilization energies are listed in Table 5. After protonation at the N-site, the N lone pair is no longer available for delocalization to X*CO(S). Therefore, the occupancy is greatly reduced and the E(2) for Lp(N) donating in X*CO(S) to JG*CO(S) diminishes. However, the occupancy of X C O ( S ) changes only slightly. This explains the previous result that the change in RCNis more important than Rco for N-protonation (3). For O(S)-protonation (5,7), a genuine C-N double bond is formed with a ~ C occupation N near 2.0 and nco(s)occupation vanishing. The drastic switch in n-electron occupancies and the corresponding changes in bond lengths for 0-protonation correspond closely to the resonance structure prescription of l a . Since the protonation is such a strong interaction, near 200 kcal mol-', it is expected to be able to drive easily such a change in n-electron structure. (I
The zwitterionic structure can enhance the rotational barrier through its C-N double-bond ~ h a r a c t e r . ~ ' -In ~ ~the present work, we would like to see if the zwitterionic resonance contribution can be tuned by protonation and hydrogen bonding at the two different sites. M of the calculated rotational barriers including those for the free molecules are collected in Table 5. There is a nearly free rotation for N-protonated molecules. The barrier heights are much lower than that for FA and TFA. In contrast, for O(S)-protonation, the barrier height is much higher than that for the free molecule. The zwitterionic structure becomes a reality as indicated by a nearly full occupancy for X C N and vanishing occupancy for n c o ( ~ as ) , shown in Table 6. In Scheme 2, the C-N double bond is assigned accordingly. Interestingly, the 0,-protonation induces an even higher C-N rotational barrier than the Orprotonation case. The former has a ZCN occupancy of 1.991 compared with 1.998 for the latter. However, the X*CN occupancy is less in the former, 0.097 compared with 0.184 in the latter. This means the net CNbonding character of the OC=O group in the free FA can be described as >C+-O-, as pointed out by Wiberg.38,39 After the protonation, it may be described as >C+-OH with not much gain in charge at C. For the protonation of TFA, carbon stays nearly neutral while sulfur gains a substantial positive charge from the proton. It may be described as >C=S Ht >C=S+-H.
+
H+
0 -0.581(-0.712) -0.478( -0.629) -0.322( -0.468) -0.3 18(-0.469) -0.499(-0.708) -0.402(-0.658) -0.556( -0.774) -0.508(-0.775)
-
two orbitals involving the E(2) for the Lp(X) n*minteraction for a different reduction in CN double-bond character. Acknowledgment. We thank the National Science Council for research funds. The computing facilities were supported by the Computer Center of National Tsing Hua University and the National Center for High-performance Computing (NCHC) in Taiwan. References and Notes (1) Burk, P.; Herodes, K.; Koppel, I. Int. J. Quant. Chem. 1993,S27,
633.
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