10490
J. Phys. Chem. 1996, 100, 10490-10496
Basicity of Acetamidine. Experimental and Theoretical Study A. I. Gonza´ lez, O. Mo´ ,* and M. Ya´ n˜ ez Departamento de Quı´mica, C-9, UniVersidad Auto´ noma de Madrid, Cantoblanco, 28049 Madrid, Spain
E. Le´ on, J. Tortajada, and J. P. Morizur Laboratoire de Chimie Organique Structural, UniVersite´ Pierre et Marie Curie, Boite 45, CNRS URA 506, 4 Place Jussieu, 75252 Paris Cedex 05, France
I. Leito,† P.-C. Maria, and J. F. Gal* Laboratoire de Chimie Physique Organique, Groupe FT-ICR, UniVersite´ de Nice-Sophia Antipolis, 06108 Nice Cedex 2, France ReceiVed: October 1, 1995; In Final Form: March 21, 1996X
The gas-phase proton affinity of acetamidine has been determined using FT-ICR spectrometry to be 230.1 kcal/mol (962.7 kJ/mol). High-level ab initio calculations were performed to investigate whether acetamidine is planar in its equilibrium conformation. The effect of solvation on both its structure and its basicity was analyzed using the self-consistent reaction field theory. The two stable isomers 1a and 1b are almost degenerate but separated by a significantly high isomerization barrier. Much higher is the tautomerization barrier between acetamidine 1a and 1,1-diaminoethylene (2). The amino rotational barrier is of the same order of magnitude as those of typically Y-conjugated systems such as guanidine. Most unexpected, this barrier is twice as large for protonated acetamide as for protonated guanidine. This is consistent with the fact that the intrinsic basicity of acetamidine is only 3.0 kcal/mol lower than that of guanidine. In solution, the situation is similar, and acetamidine is slightly less basic than guanidine. Similarly, 1,1-diaminoethylene is predicted to be a very strong carbon base in the gas phase with a proton affinity 6 kcal/mol greater than that of guanidine.
Introduction Many efforts have been devoted to the study of cationization reactions of monofunctional compounds in the gas phase.1 However, molecules that exhibit several active sites are the rule rather than the exception in chemistry. In this respect, particular attention was devoted, from both experimental and theoretical points of view, to different aspects of the chemistry of amides.2 Quite surprisingly, however, and despite their biochemical and medicinal activity,3 information on the properties of amidines is rather scarce.4 Our research groups have focused lately their attention in the study of this particular subset of bidentate bases, because, among other reasons, they are expected to be strong bases in the gas phase. In a previous study5 we have predicted formamidine to be a strong base in the gas phase, by means of high level ab initio calculations. Unfortunately, we have not been able to generate neutral formamidine in the gas phase from an equilibrium proton transfer study; hence, an experimental confirmation of these theoretical predictions was not attainable. This prompted us to investigate the gas-phase basicity of the next member of the series, acetamidine 1, which is experimentally accessible. Since, to the best of our knowledge, there is an almost complete lack of data regarding this molecule, we have considered it of interest to investigate not only its intrinsic basicity but other structural, conformational, and reactivity problems. Of particular interest will be the comparison of the nonplanarity of the amino groups of acetamidine and acetamide, to investigate whether the effects of a CdNH group or a carbonyl group are similar. Since the protonated species is a conjugated system, the influence of resonance phenomena on † Permanent address: Institute of Chemical Physics, Tartu University, EE 2400 Tartu, Estonia. X Abstract published in AdVance ACS Abstracts, May 1, 1996.
S0022-3654(95)03042-5 CCC: $12.00
the acetamidine gas-phase basicity will deserve also some attention, as well as similar effects on the corresponding rotational barriers. The intrinsic properties of its isomer 1,1diaminoethylene (2) will be also studied. Since we can obtain a reliable value for the experimental proton affinity of acetamidine and since its size permits us to employ high-level ab initio techniques, this system constitutes also a suitable benchmark case to investigate the reliability of alternative theoretical formalisms, in particular in the framework of the density functional theory, to reproduce these intrinsic properties. This information may be of a great value in our future work, in which investigation of the intrinsic properties of gradually larger bases is envisaged. For the sake of completeness we shall also explore the influence of different solvents on acetamidine’s structure, harmonic vibrational frequencies, and basicity. Experimental Section Materials. Acetamidine was obtained as a free base from the corresponding hydrochloride salt (Aldrich) by reaction under nitrogen with sodium methoxide freshly prepared from sodium and methanol as reported by Bordwell and Ji.6 The free bases were sublimated twice in vacuo (10-2 Pa). Physical Measurements. Relative basicities for acetamidine (∆GB values) are reported in Table 1. They were obtained from proton-transfer equilibrium constants measured against pertinent references using Fourier transform ion cyclotron resonance (FTICR) spectrometry, as described previously.7 Sensitivities (Sr, relative to N2) of the ionization gauge were estimated according to the method of Bartmess and Georgiadis:8 Sr ) 0.36R(ahc) + 0.30, where R(ahc) is the average molecular polarizability based on atomic hybrid components calculated using the © 1996 American Chemical Society
Basicity of Acetamidine
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TABLE 1: Experimental Gas-Phase Basicity of Acetamidine (in Kilocalories per Mole) ref (n-Pr)2NH 1-Me-pyrrolidine (i-Pr)2NH PhCH2NMe2
GB (B ) PA (B ) GB(Ref)a ∆GB(338 K)b acetamidine)c acetamidine)d 219.7 220.9 222.0 222.0
1.91 ( 0.02 0.81 ( 0.02 0.14 ( 0.02 0.10 ( 0.02
221.9
230.1 (229.7)e
a At 298.15 K: Lias, S. G.; Liebman, J. F.; Levin, R. D. J. Phys. Chem. Ref. Data 1984, 13, 695-808; addition and corrections: personal communication, 1987. b Gibbs energies for the reaction BH+ + Ref h RefH+ + B (1); quoted uncertainties correspond to the standard deviation for three to four measurements. c Absolute Gibbs energies (298.15 K) for the reaction BH+ f B + H+ (2); mean value, no temperature correction. d PA(B) ) GB(B) + T∆S; ∆S ) S(H+) + S(B) - S(BH+); S(H+) ) 26.039 ( 0.002 cal K-1 mol-1. Chase, M. W.; Davies, C. A.; Downey, J. R., Jr.; Frurip, D. J.; McDonald, R. A.; Syverud, A. J. Phys. Chem. Ref. Data 1985, 14, Suppl. 1; S(B) S(BH+) ≈ -R ln σB/σBH+ (-R ln(1/2) for acetamidine). e PA value obtained when the ∆S term is taken from the corresponding ab initio calculations.
additivity scheme of Miller.9 At the beginning of the introduction of these very low vapor-pressure samples in the analyzer region of the spectrometer, relatively long pressure drifts were observed. Mass spectra revealed the presence of CO2, H2O, and small amounts of the respective amides. Measurements were undertaken when less than 10% of impurities were present. The proton affinity (PA) has been estimated from GB using the proton translational entropy and the change in symmetry number as indicated in Table 1. Computational Details Standard ab initio molecular orbital calculations have been performed using the Gaussian 9210 and Gaussian 92DFT11 series of programs. Geometry optimizations for all structures were carried out at the MP2/6-31G* level. Harmonic vibrational frequencies were evaluated using analytical second derivatives at the same level. To investigate further the effects of the basis set on the optimized structure of acetamidine and, in particular, on the degree of pyramidalization of the amino group, we have also performed geometry optimizations at the MP2 level with 6-311G(d,p), 6-311+G(d,p), and 6-311++G(2df,p) basis sets. The corresponding optimized geometries will be compared with those obtained by using two different density functional approaches, namely SVWN and BLYP. In the first one the exchange functional is Slater (S), corresponding to the freeelectron gas,12 while the correlation part corresponds to the Vosko, Wilk, and Nusair (VWN) parametrization of exact uniform gas results.13 In the second one, the exchange functional is Becke (B), which includes a gradient correction,14 and the correlation part is given by the gradient-corrected functional of Lee, Yang, and Parr (LYP).15 In both cases the 6-31G* basis was used for orbital expansion. The effect of solute-solvent interactions was taken into account via the self-consistent reaction field (SCRF) method, based on Onsager’s theory of solvation.16 In this approach the solute is enclosed in a spherical cavity of radius a0 surrounded by the solvent, which is considered as a uniform dielectric with a given relative permittivity, r. The stabilization of the system is the result of the interaction between the dipole moment of the solute with the dipole induced in the surrounding medium. For the particular cases of protonated species it is necessary to add the corresponding monopole interaction. In the SCRF formalism the solute-solvent interaction is treated as a perturbation, and the cavity radius was fixed during geometry optimizations. In our survey we have included the following
solvents: cyclohexane (r ) 2.6), dichlorobenzene (r ) 9.93), ethanol (r ) 24.3), acetonitrile (r ) 37.5), and water (r ) 78.54). Geometry optimizations were carried out at the HF/631+G(d,p) level, while final energies were calculated at the MP2/6-311+G(2df,2p) level. To have as reliable energetics as possible, the total energies of acetamidine 1 and 1,1-diaminoethylene (2), as well as those of their protonated species and those of the most relevant transition states, were obtained in the framework of the G2 theory.17 G2 theory is a composite procedure based on MollerPlesset perturbation theory at second and fourth orders (MP2 and MP4) and quadratic configuration interaction including single-, double-, and triple-excitation (QCISD(T)) levels of theory. In the G2 procedure a total energy effectively of a QCISD(T)/6-311+G(3df,2p) quality is obtained by assuming additivity of different basis set enhancements at the MP4 level and additivity of basis set and correlation effects between MP4 and QCISD(T). This theoretical scheme has been reported17-19 to yield ionization energies, atomization energies, proton affinities, and heats of formation in agreement with the experimental values within (0.1 eV. These results will be compared with those obtained by using a more economic version of the G2 theory, usually referred to as G2(MP2)20 theory, by which the basis set extension energy corrections are obtained at the second-order Møller-Plesset (MP2) level, and which may represent a sensible alternative for the treatment of much larger systems. Actually, it has been shown31 that, in general, the average absolute deviation of the G2(MP2) theory is 1.58 kcal/mol, compared to 1.21 kcal/mol for the G2 theory. Furthermore, in a recent study of Li+, Na+, Mg+, and Al+ complexes of formamidine,5 we have found that the G2(MP2) scheme yields relative energies in agreement with the G2 formalism within (0.2 kcal/mol. Although in the original G2 and G2(MP2) schemes, the zero point energy (ZPE) corrections are evaluated at the HF/6-31G* level, in this work, as mentioned above, the harmonic vibrational frequencies were obtained at the MP2/6-31G* level and the ZPEs were scaled by the empirical factor 0.93. We have considered it also of interest to investigate whether the use of DFT optimized structures rather than the MP2 optimized ones has a significant influence on the G2 energies. In what follows we shall identify this approach as G2(DFT). Similarly, the G2 and G2(MP2) estimated proton affinities will be compared with those obtained by using SVWN and BLYP functionals with a 6-311+G(3df,2p) basis set, which is the largest orbital expansion used in G2 theory. For these latter calculations the corresponding DFT/6-31G* geometries were used. Results and Discussion Structures, Rotational and Isomerization Barriers. The total energies of the systems under investigation, at the different levels of accuracy considered in this work, are reported in Table 2. This table includes also the G2 results corresponding to the transition states between forms 1a and 1b (TS1a1b) and between 1a and 2 (TS1a2), as well as those corresponding to the rotation of the amino group in both neutral and protonated acetamidine (TSrot and TSrotH+, respectively). The most stable conformer of acetamidine 1a has the N-H imino bond trans with respect to the amino group (see Figure 1). The cis conformer 1b lies only 1.0 kcal/mol above. The preference for the former conformation is likely due to more favorable nonbonding interactions involving the imino nitrogen lone pair and the amino hydrogens. Actually, there are some significant differences between the geometries of both conformers. Both present a similar N3C1C4 bond angle, but while in
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TABLE 2: Total Energies (Hartrees) of the Systems Included in This Study G2(MP2) 1a 1b 2 1H+a 1H+b 2H+ TS1a1b TS1a2 TSrot TSrotH+ H2CdNH H2CdNH2+
-188.98454 (233.2)a -188.98298 -188.96855 -189.35417 -189.30687 -189.29736 -188.94312 -188.88543 -188.96079 -189.32005 -94.43112 (206.1) -94.75977
G2
G2(BLYP)
G2(SVWN)
BLYP/ 6-311+G(3df,2p)
SVWN/ 6-311+G(3df,2p)
-188.98915 (233.2) -188.97672 (233.5) -188.97639 (232.5) -189.32518 (232.9) -188.38603 (230.2) -188.98761 -188.97513 -188.97470 -188.97317 -189.35884 -189.34507 -189.34481 -189.70777 -188.76413 -189.31137 -188.94758 -188.88971 -188.97401 -189.32467 -94.46327 (206.1) -94.79183
a The estimated proton affinities including thermal corrections of acetamidine 1a and methylenimine (H2CNH) are given within parentheses in kilocalories per mole.
Figure 1. MP2/6-31G* optimized structures. Bond lengths are in angstroms and bond angles in degrees.
the cis conformer the C4C1N2 bond angle is about 7° smaller than the N3C1N2 one, in the trans isomer the latter is 9° smaller than the former. This implies that in both 1b and 1a there is a stabilizing interaction between the imino lone pair and a hydrogen of the methyl group, in the first case, and a hydrogen of the amino group in the second. Since the amino hydrogens bear a greater positive charge than the methyl hydrogens, the stabilizing interaction is slightly larger in the trans than in the cis conformer. This seems to be consistent with the fact, mentioned above, that in the trans isomer the difference between the N3C1N2 and the C4C1N2 bond angles is 2° greater than in the cis one. Consequently, the N2‚ ‚ ‚H6 distance in 1a is almost 0.1 Å shorter than the N2‚ ‚ ‚H8 distance in 1b. On the other hand, the different orientation of the imino group in the trans conformer seems to favor a better conjugation of the amino nitrogen lone pair with the imino π-system. This is mirrored
in an amino rotational barrier surprisingly high (9.3 kcal/mol at the MP2/6-31G* level and 9.5 at the G2 level, 1 cal ) 4.184 J) if one takes into account that in a typical Y-conjugated system such as guanidine, the MP2/6-31G* amino rotational barrier was predicted21 to be 6.2 kcal/mol. Furthermore, the rotation of the NH2 group is accompanied by a sizeable (0.07 Å) increase in the C1N3 bond length. A similar effect is found for the amino rotation in the most stable protonated species, which leads to a C1N3 bond lengthening of 0.09 Å. It should also be noted that in both neutral and protonated acetamidine amino rotation is accompanied by an increase of the NH2 pyramidalization (see Figure 1). However, while in the neutral species the NH2 group bends away from the methyl group and toward the NH group to disfavor the repulsive interaction between both nitrogen lone pairs, in the protonated species the pyramidalization bends the NH2 group toward the methyl group to favor the attractive
Basicity of Acetamidine
J. Phys. Chem., Vol. 100, No. 24, 1996 10493
TABLE 3: Geometrical Parameters Obtained at Different Levels of Theory for System 1aa
C1N2 C1N3 C1C4 N2H5 N3H6 N3H7 C4H8 C4H9 C4H10 N3C1N2 C4C1N2 H5N2C1 H6N3C1 H7N3C1 H8C4C1 H9C4C1 H10C4C1 C4C1N2N3 H5N2C1N3 H6N3C1N2 H7N3C1H6 H8C4C1N2 H9C4C1H8 H10C4C1H8 A B C a
HF/6-31G*
MP2/6-31G*
MP2/ 6-311G(d,p)
MP2/ 6-311+G(d,p)
MP2/ 6-311++G(2df,p)
QCISD/ 6-311G(d,p)
BLYP/ 6-31G*
SVWN/ 6-31G*
1.258 1.380 1.511 1.002 0.998 0.996 1.082 1.085 1.086 119.7 126.4 111.4 113.6 117.0 111.1 110.0 110.5 -177.2 175.7 13.7 135.6 0.0 120.8 -120.3 10436.18 9256.60 5079.57
1.286 1.389 1.508 1.022 1.015 1.013 1.091 1.093 1.095 118.8 127.3 109.7 112.3 116.0 110.8 110.1 110.8 -175.4 174.4 14.5 132.2 -5.0 120.8 -120.2 10232.96 9121.42 4998.63
1.283 1.389 1.510 1.018 1.011 1.009 1.091 1.093 1.095 119.1 127.1 108.8 112.2 115.3 110.8 109.9 110.4 -176.0 174.7 16.0 131.5 -0.5 120.9 -120.3 10252.40 9170.08 5017.16
1.285 1.387 1.509 1.018 1.011 1.009 1.090 1.093 1.094 119.1 126.9 109.2 113.5 116.3 110.7 109.9 110.5 -175.9 175.2 15.9 135.3 -2.9 120.6 -120.3 10233.41 9175.43 5009.78
1.279 1.375 1.500 1.017 1.010 1.007 1.088 1.091 1.092 119.2 126.7 109.9 114.9 118.0 110.9 110.3 110.6 -177.1 176.0 13.3 142.0 -1.7 120.7 -120.3 10326.70 9280.86 5056.20
1.280 1.397 1.516 1.018 1.011 1.010 1.093 1.096 1.097 119.3 127.0 109.2 111.8 114.6 110.8 109.8 110.3 -176.1 174.9 16.4 129.4 -0.9 120.9 -120.4 10211.05 9115.32 4994.26
1.295 1.401 1.529 1.031 1.022 1.021 1.104 1.100 1.106 118.8 127.2 109.6 112.8 116.5 110.8 110.8 111.1 -176.4 174.1 14.7 133.4 -0.9 119.2 -120.8 10233.09 9121.55 4998.69
1.282 1.370 1.498 1.029 1.021 1.018 1.104 1.101 1.106 119.0 127.0 109.6 114.1 118.7 110.8 110.9 111.1 -177.3 175.0 12.0 142.3 -0.5 119.2 -120.9 10336.64 9257.00 5054.90
Bond distances in angstroms, angles in degrees, and rotational constants (A, B, C) in megahertz.
interaction between the lone pair of this amino group and one of the hydrogen atoms of the other amino group (that which remains in the plane of the molecule). We have also found that the G2 energy difference between TS1a1b and the global minimum 1a is 26.1 kcal/mol, so, in principle, forms 1a and 1b may be experimentally accessible. The isomerization process involves the rotation of the imino group, which, according to our results, is accompanied by a significant change in the C1N2H5 angle, so that in the corresponding transition state, the imino hydrogen is only 14° out of the plane of the molecule. 1,1-Diaminoethylene (2) is predicted to be 10 kcal/mol less stable than the global minimum 1a, the isomerization barrier being 62.4 kcal/mol high. The structures of all these neutrals as well as those of their protonated species are given in Figure 1. Although the structure of 2 has been reported at the same level of accuracy in the literature,21 we include it in Figure 1 for the sake of a better discussion of protonation effects. It is worth recalling, as indicated in ref 21, that 1,1-diaminoethylene (2) has C2 symmetry, while the C2V planar structure is a thirdorder saddle point, with transition vectors displacing the amino and the methylene hydrogens out of the molecular plane. However, in the most stable protonated species (1H+a), both amino groups lie in the plane of the molecule, to enhance the conjugation of both lone pairs. As we shall discuss later, this fact will be reflected in both the value of the proton affinity of acetamidine and the NH2 rotational barrier in the cation. Protonation at the amino nitrogen leads to a much less stable species 1H+b. The values of the H6N3C1N2 and H7N3C1H6 dihedral angles (see Table 3) clearly show that, at the HF/6-31G* level, the NH2 group of acetamidine deviates significantly from planarity. However, in contrast with what was found at the same level for acetamide,2n one of the methyl hydrogens lies in the plane of the molecule. Since a good choice of the basis set may be crucial for a proper description of the planarity of the
amino group and the distortion of the methyl group, we have explored the performance of several basis sets. All of them were built from the 6-311G basis, which was specially designed for correlation calculations.22 The results have been summarized in Table 3. At all levels, the amino group exhibits a large deviation from planarity, with slight changes when the basis set is enlarged. Table 3 shows also the results obtained using the BLYP and the SVWN functionals when a 6-31G* basis set is used for the wave function expansion. In general, an inspection of bond lengths and bond angles shows that both functionals yield structures in good agreement with the MP2 ones. When this comparison is made in terms of the corresponding rotational constants, which is a tight criterion to compare geometries,23 the best choice seems to be the SVWN functional. As shown in Table 3, the SVWN rotational constants deviate from the MP2/6-311++(2df,p) by 0.1% on average, while the BLYP ones deviate by more than 10 times this value. We have considered it of interest to investigate the nonplanarity of acetamidine from an energetic point of view. For this purpose we have optimized the structure of this neutral by forcing the amino group to be planar. These optimizations have been carried out at both the MP2/6-31G* and the MP2/6311+G(d,p) levels. In both cases the planar structure was predicted to be a transition state, with one imaginary frequency (474i and 605i cm-1, respectively). At both levels the planar structure is found to be 1.2 kcal/mol less stable than the nonplanar conformer 1a. Nevertheless, when the energy of this planar structure is evaluated at the G2 level (without including the ZPE correction), both the planar and the nonplanar conformers become almost degenerate (the energy difference is 0.52 kcal/mol). Hence, we may conclude that, very likely, the planar species prevail at room temperature. An inspection of the different contributions to the G2 energy indicates that those from high angular momentum basis at fourth order (MP4/6-311G(2df,p)) favor the planar conformer with respect to the nonplanar one.
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TABLE 4: Harmonic Vibrational Frequencies (in cm-1) for the Most Stable Conformer of Acetamidine 1a and Its Protonated Form system 1aH+
system 1a MP2/6-31G* 3701 (3330) 3580 (3226) 3516 (3429) 3219 3189 3107 1758 (1650) 1679 (1608)
a
SVWN/6-31G* 3614 3492 3418 3104 3062 2989 1737 1585
1551 (1368) 1541 (1429) 1487 (1460)
1450 1435 1430
1439 1204 (1192) 1173 (1124) 1095
1340 1136 1121 1027
1034 (1044) 881 (1002) 875 692 545 534 441 405 113
964 868 814 601 520 496 420 362 76
a
assignment
MP2/6-31G*
SVWN/6-31G*
NH2 a-stretch NH2 s-stretch NH stretch CH stretch CH stretch CH stretch N-C-N s-stretch N-C-N s-stretch + NH2 sciss CH3 s-def CH3 def N-C-N s-stretch + NH bend + CH3 def CH3 def umb N-H bend NH2 rock C1 pyram + CH3 def CH3 rock C-C stretch N-H bend out of plane NH2 wag N-C-N bend NH2 pyram N4-C1-C5 bend NH2 twist CH3 rock
3709 3707 3659 3580 3242 3220 3126 1791
3590 3589 3491 3477 3122 3093 3012 1755
NH2 a-stretch ip NH2 a-stretch op NH2 s-stretch ip NH2 s-stretch op CH3 stretch CH3 stretch CH3 stretch CN2 a-stretch
1738 1640 1582
1660 1562 1527
1537 1518 1468 1155
1438 1415 1368 1107
1117 1106 1046 904 728 597 547 537 503 500 443 78
1057 1027 979 904 697 584 530 520 485 466 431 89
NH2 sciss ip NH2 sciss op N-C-N s-stretch + C-C stret CH3 def CH3 def CH3 def umb NH2 rock op + CN2 s-stretch NH2 rock ip C1 pyram CH3 rock + NH2 rock C-C stretch NH2 twist ip NH2 wag ip NH2 wag op N-C-N bend NH2 rock ip NH2 twist op N-C-C bend torsion
assignment
Experimental values (taken from ref 26) for neutral acetamidine in the condensed phase are given in parentheses.
Species 1H+a is also the result of protonation of 1,1diaminoethylene (2) at the carbon atom. Since 2 is already a conjugated system, this analysis offers an alternative viewpoint to understand the charge redistributions upon protonation of acetamidine. Protonation at the carbon atom of 2 implies necessarily a charge transfer from the base to the bare proton, which results in a certain depopulation of the C1-C4 bond and in an increase of the electronegativity of the central carbon C1. As a consequence, it polarizes the C1-N2 and C1-N3 bonds, whose charge density increases with respect to the neutral system. In summary, we can conclude that protonation implies an enhancement of the resonant charge at the C(NH2)2 moiety. This is consistent with a significant increase of the amino rotational barrier upon protonation, which becomes 21.4 kcal/ mol high and which is much larger than that measured for protonated guanidine.24 An increase of the NH2 rotational barrier should be expected in the protonated species since the amino rotation implies the breaking of the resonance in the C(NH2)2 moiety. However, in guanidinium ion after the rotation there remains the conjugation between the other two amino groups, which is responsible for the still high stability of the system. Consistent with this, the energy required to rotate the first amino group in guanidinium ion is moderately low (∼13 kcal/mol), while that required to rotate the second amino group is more than 3 times greater.25 As we shall explain later, this seems to be in agreement with the small increment observed in the gas-phase basicity of guanidine with respect to acetamidine. Harmonic Vibrational Frequencies. In Table 4 we present the harmonic vibrational frequencies and their assignments for neutral and protonated acetamidine. To the best of our knowledge, only the infrared spectrum of acetamidine in the condensed phase has been reported26 in the region 450-4000 cm-1. In this spectrum of acetamidine (liquid) a broad absorption with three centers at 3429, 3330, and 3226 cm-1
was observed, which were assigned to the ν(NH), νas(NH2), and νsym(NH2) vibrations, respectively. These values are in fairly good agreement with our calculated harmonic frequencies (see Table 4). However, our results indicate that, in the gas phase, the N-H stretching should be observed at lower frequencies than the NH2 stretching displacements, in contrast with the experimental assignments, probably due to the fact that the vibrations associated with the amino and imino groups are strongly affected by intermolecular interactions. Likely due to the same reasons, our results predict the N-H bending displacement to appear at lower frequencies than the CH3 deformation vibrations, again in contrast with the experimental assignments for liquid acetamidine. The remaining vibrational frequencies are in reasonably good agreement with the experimental values, in particular the two N-C-N and the C-C stretching vibrations as well as the NH2 and CH3 rocking displacements. This agreement is reasonably good for both MP2/6-31G* and SVWN/ 6-31G* values, being slightly better for the latter. With regard to the vibrational modes of protonated acetamidine, it may be observed that, due to its symmetry, the displacements involving both amino groups appear as in-phase (ip) and out-of-phase (op) combinations. Protonation Energies. Acetamidine is a strong base in the gas phase (see Table 2). Its proton affinity is merely 3 kcal/ mol lower than that of guanidine,25 one of the strongest organic bases. The high basicity of acetamidine is, in some sense, an unexpected result, since the high basicity of guanidine is usually explained in terms of electronic resonance favored by the high symmetry of the guanidinium ion, while in protonated acetamidine the electronic resonance is restricted to two amino groups. This problem deserves a closer analysis since there was some controversy21,27 regarding the role of the electronic resonance on the properties of Y-conjugated systems.
Basicity of Acetamidine The gas-phase proton affinity of methylenimine (H2CdNH) has been recently reported28 to be 204.1 kcal/mol. Our G2 results are in very good agreement with this experimental value (see Table 2). Although the experimental proton affinity of formamidine is not known, we can take the G2 result (225.4 kcal/mol)5 as an accurate estimated value. The conclusion would be that formamidine is about 20 kcal/mol more basic than methylenimine due to the enhanced stability of the conjugated formamidinium ion. The extension of the electronic resonance to a third amino group, on going from formamidine to guanidine, implies, however, a much smaller increase (∼7.6 kcal/mol) in the gas-phase basicity of the system. This nonadditivity effect is likely due to the steric hindrance of planarity in protonated guanidine,21 which would decrease the stabilization of the system by resonance, and also to a saturation of the resonance effect. This would be consistent with the fact mentioned above that the NH2 rotational barrier for protonated guanidine is sizeably lower than that predicted for protonated acetamidine. The proton affinity of acetamidine is intermediate between those of formamidine and guanidine, but closer to that of the latter. This reflects a basicity increase by methylation which is typically29 of about 5-6 kcal/mol, most of it due to the high polarizability of the methyl group. We might then conclude that resonance stabilization is not negligible as revealed by the fact that the NH2 rotational barriers of the protonated species are larger than those of the neutrals. However, the enhancement effect of the third amino group on the basicity of the system is 3 times smaller than that produced by the second amino group and, therefore, only slightly higher than the enhancement achieved by methyl substitution. As a consequence, the basicity of acetamidine is surprisingly close to that of guanidine. It may be also observed that the experimental proton affinity is fairly well reproduced at the G2 level of theory. Noteworthy, the same values are obtained, within 0.05 kcal/mol, at the G2 and the G2(MP2) levels. This clearly shows, together with similar findings reported in the literature,5,30 that the more economic scheme is very well suited for this kind of calculation. Also interestingly, the agreement between G2 calculations and experimental values slightly increases when the former are performed on DFT optimized structures rather than on MP2 optimized ones. The agreement is similarly good when DFT formalisms are used provided that a 6-311+G(3df,2p) basis set is employed, although BLYP values are closer to the G2 ones than those obtained when the SVWN functional is used. Influence of the Solvent. The solvent effects on the structure of acetamidine are presented in Table 1 of the supporting information. Similarly to what was found for acetamide,2n the nonplanar structure is always preferred for either the slightly or strongly polar solvents. Actually, the nonplanarity of the amino group is practically unaffected by solvent interactions, while only the distortion of the methyl group out of the molecular plane slightly increases. It must be indicated that in our survey we have included solvents with a relative permittivity equal to water. However, these results cannot be taken as reliable in such a medium, since in our model we neglect all specific solvation effects through the formation of hydrogen bonds with the solvent. Nevertheless, very likely, in acetamidine, as it was found for acetamide,2n these hydrogen bonds would not alter significantly the nonplanarity of the system. To have a more precise idea on the energy difference between planar and nonplanar conformations, we have also optimized the former at the HF/6-31+G(d,p) level and for a solvent with r ) 78.54. This optimization showed that, as for the isolated molecule, the planar conformer is a transition state, which lies
J. Phys. Chem., Vol. 100, No. 24, 1996 10495 0.2 kcal/mol above the nonplanar one. When the energy of both conformers is evaluated at the MP2/6-311+G(3df,2p) level, this energy difference becomes 0.4 kcal/mol. Hence, we may conclude that also, in solution, very likely the planar conformer prevails. The harmonic vibrational frequencies predicted for the most stable conformer in aqueous solution are given in Table 2 of the supporting information. A comparison with the values obtained for the gas phase indicate that the NH stretchings undergo a blue-shifting of ca. 9 cm-1, while the C-NH stretching is red-shifted by ca. 30 cm-1. Similarly, the NH2 rocking is shifted 8 cm-1 toward greater frequencies, while the bending of the N-H group out of plane is shifted 17 cm-1 to lower frequencies. The greatest displacement corresponds to the pyramidalization of the amino group, which in solution appears at frequencies 42 cm-1 lower than in the gas phase. Since, as indicated above, the specific solute-solvent interactions through the formation of hydrogen bonds are not taken into account in the SCRF model, we have estimated the basicity of acetamidine in aqueous solution by means of the following process:
(acetamidine)solv + (NH4+)solv f (acetamidineH+)solv + (NH3)solv (1) We may reasonably assume that specific solvation effects are similar in both sides of reaction 1. These calculations have been carried out at the SVWN/6-311+G(3df,2p) level on structures optimized at the SVWN/6-31+G(d,p) level. Our results show that acetamidine is still a strong base in condensed media since its basicity (in ∆G) is 12 kcal/mol higher than that of ammonia. It must be noted that this is equivalent to 8.8 pK units, in fairly good agreement with the experimental differences between the basicities of both systems in aqueous solution (7.7 pK units).31 Using this model, our estimates also show acetamidine to be 1.0 pK unit less basic than guanidine, again in reasonably good agreement with the experimental difference (1.2).31 The fairly good agreement between these theoretical estimates and the experimental values seems to confirm that our assumption that specific solute-solvent interactions will not be significantly different for both sides of reaction 1 is acceptable. Conclusions We have found that when high-level ab initio calculations are performed, the planar and nonplanar conformations of acetamidine are almost degenerate. Hence, we may reasonably conclude that the planar form would prevail at room temperature. This situation does not change significantly in condensed media. The two stable nonplanar isomers 1a and 1b are almost degenerate but separated by a significantly high isomerization barrier. Much higher is the tautomerization barrier between acetamidine 1a and 1,1-diaminoethylene (2). Quite interestingly, the amino rotational barrier is of the same order of magnitude as those of typically Y-conjugated systems such as guanidine. For protonated acetamidine the rotational barrier is twice as large as that for protonated guanidine. This is consistent with the fact that although the charge delocalization is maximum in guanidinium ion, the intrinsic basicity of guanidine is only 3.0 kcal/mol greater than that of acetamidine. These results seem to indicate that the high basicity of these systems is mainly due to the existence of the amidine function. Actually, formamidine was predicted to be quite a strong base in the gas phase, the greater basicity of acetamidine being a typical methyl substitution effect. However, the extension of
10496 J. Phys. Chem., Vol. 100, No. 24, 1996 the resonance to a third amino group has a smaller effect on the basicity as well as on the amino rotational barrier, due to the saturation of the resonance stabilization of the system. On the other hand, 1,1-diaminoethylene is predicted to be a very strong carbon base in the gas phase, with a proton affinity 6 kcal/mol greater than that of guanidine. In solution acetamidine is predicted to be only 1.0 pK unit less basic than guanidine, in good agreement with experimental evidence. This agreement may be taken as an indication that a SCRF model may yield reliable basicities in solution provided that the specific interactions of reactants and products with the solvent, in the corresponding acid-base reaction, are similar, as seems to be the case for reaction 1. There is a very good agreement between the G2 and the G2(MP2) calculated proton affinities and the experimental ones. At least for the system investigated, the agreement is similarly good when DFT approaches are employed, provided that the 6-311+G(3df,2p) basis set is used for the expansion. Hence, DFT calculation at this level might be an economic alternative to obtain accurate estimates of proton affinities of large systems, the treatment of which at the G2 level may be too expensive. Acknowledgment. This work has been partially supported by the DGYCIT Project PB93-0289-C02-01 and by the Accio´n Integrada Hispano-Francesa 302B. Supporting Information Available: Solvent effects on the structure and harmonic vibration frequencies of acetamide (2 pages). Ordering information is given on any current masthead page. References and Notes (1) Taft, R. W. Prog. Phys. Org. Chem. 1983, 14, 247. (2) (a) Davies, M.; Hopkins, L. Trans. Faraday Soc. 1957, 53, 1563. (b) Klotz, I. M.; Franzen, J. S. J. Am. Chem. Soc. 1962, 84, 3461. (c) Barone, G.; Rizzo, E.; Vitagliano, V. J. Phys. Chem. 1970, 74, 2230. (d) Finer, E. G.; Franks, F.; Tait, M. J. J. Am. Chem. Soc. 1972, 94, 4424. (e) Cox, R. A.; Druet, L. M.; Klausner, A. E.; Modro, T. A.; Wan, P.; Yates, K. Can. J. Chem. 1981, 59, 1568. (f) Lovas, F.; Suenram, R. D.; Fraser, G. T.; Gillies, G. W.; Zozom, J. J. Chem. Phys. 1988, 88, 722. (g) Barone, G.; Castronuovo, G.; Vecchio, P. D.; Elia, V.; Giancola, C. Thermochim. Acta 1987, 122, 105. (h) Radom, L.; Lathan, A.; Hehre, W. J.; Pople, J. A. Aust. J. Chem. 1972, 25, 1601. (i) Hagler, A. T.; Leiserowitz, L.; Tuval, M. J. Am. Chem. Soc. 1976, 98, 4600. (j) Fogarasi, G.; Pulay, P.; Torok, F.; Boggs, J. E. J. Mol. Struct. 1980, 69, 79. (k) Jeffrey, G. A.; Houk, K. N.; PaddonRow, M. N.; Rodan, N. G.; Mitra, J. J. Am. Chem. Soc. 1985, 107, 321. (l) Lim, K. T.; Francl, M. M. J. Phys. Chem. 1987, 91, 2716. (m) Wong, M. W.; Wiberg, K. B.; Frisch, M. J. J. Am. Chem. Soc. 1992, 114, 1645. (n) Wong, M. W.; Wiberg, K. B. J. Phys. Chem. 1992, 96, 668. (o) Scheiner, S.; Wang, L. J. Am. Chem. Soc. 1993, 115, 1958. (p) Wen, N.; Brooker, M. H. J. Phys. Chem. 1993, 97, 8608. (q) Floria´n, J.; Johnson, B. G. J.
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