J. Phys. Chem. 1995,99, 13890-13898
13890
Potential Energy Surface of Protonated Formamide and of Formamide-X+ (X = Li, Na, Mg, and AI) Complexes J. Tortajada,*t+E. Leon,+JPP. MorizurJ A. Luna; 0. M6,*7* and M. YBiied Laboratoire de Chimie Organique Structurale, Universit; Pierre et Marie Curie, Boite 45, CNRS URA 506, 4 Place Jussieu, 75252 Paris Cedex 05, France, and Departamento de Quimica, C-9, Universidad Autbnoma de Madrid, Cantoblanco, 28049-Madrid, Spain Received: April 14, 1995; In Final Form: July 12, 1995@
High-level ab initio calculations have been performed to investigate the potential energy surfaces corresponding to the protonation of formamide (HCONH2; 1) and to its association to different metal monocations, namely, Li+, Na+, Mg+, and Al+. The structures, relative stabilities, and gas-phase reactivities of its tautomers, formamidic acid (HN=CHOH; 2) and (aminohydroxy)carbene (H2NCOH; 3) were also investigated. The unimolecular reactions of protonated formamide are discussed, in particular those corresponding to loss of water, loss of ammonia, and loss of carbon monoxide. We have found that in the latter two cases the mechanisms are essentially govemed by the high basicity of ammonia with respect to carbon monoxide. The oxygen atom is the most basic center not only for proton attachment but also for metal monocation association. Tautomerization processes connecting the three isomers of formamide are not catalyzed by cationization with the aforementioned metal monocations.
-
Introduction The last two decades have witnessed a great development of ion chemistry in the gas-phase. Some significant advances in this area were the result of combining different experimental techniques and ab initio molecular orbital calculations on different aspects of the corresponding potential energy surfaces (PES's).l-Io As a consequence many reaction mechanisms have been unambiguously established and many new species were either generated or identified by means of neutralizationreionization experiments.I0 Particular attention was devoted to organometallic complexes due to their importance in different fields such as homogeneous and heterogeneous catalysis and metal-organic chemical vapor decomposition or as agents capable of stabilizing neutral compounds of low stability. This latter possibility is particularly relevant when the neutral is a bidentate base which might favor the formation of chelates where the metal cation interacts with both basic centers simultaneously. These chelates are very often responsible for anomalously high binding energies. This is the case, for instance, of some azoles" and azine^'^^'^ when they react in the gas-phase with alkali metal monocations, via the formation of nonclassical bridged structures. Similar structures also explain the enhanced stability of some complexes between Li+ and nitrogen-containing three-membered rings.I4 Formamide constitutes a paradigmatic example of a bidentate base, and it was the subject of many theoretical and experimental s t ~ d i e s . ' ~A - ~majority ~ of these studies were motivated by the fact that formamide is the simplest amide containing a prototype HNC=O peptide linkage and can be used as a model to understand proton exchange processes in peptides and proteinsz7or the hydrolysis of peptide bondsz8in living systems. Furthermore, formamide (1) presents two tautomeric forms, namely, formamidic acid (HN=CHOH; 2) and (aminohydroxy)carbene (HzNCOH; 3), and the formamide-formamidic acid tautomerizationz'~23~29~30 has also been used to model tautomerization in larger bases, in particular in connection with the
' UniversitC Pierre et Marie Curie.
* Universidad Authoma de Madrid.
Abstract published in Advance ACS Abstracts, September 1, 1995.
guanine and uracil systems. The energy barrier for the 1 2 isomerization was recently estimated by Wang et a1.21to be 48.9 kcallmol. But more importantly, in the same paper it was shown that interaction with a single H20 molecule reduces this barrier to 22.6 kcallmol. Estimations for the isomerization barriers for 1 3 and 2 3 (77 and 86 kcallmol, respectively) have also been reported very recently in the literature.I0 The recent generation of formamidic acid (2)and (aminohydroxy)carbene (3) in the gas-phase by one-electron reduction of the corresponding radical cations'O prompted us to investigate their intrinsic reactivities versus different monocations and to study whether the association with Li+, Naf, Mg+, or Al+ may catalyze the 1 2, 1 3 and 2 3 isomerizations in the gas-phase. Significant aspects of their gas-phase reactivity will be defined by their gas-phase protonation. Although protonation of formamide has been extensively studied, to the best of our knowledge, a complete description of the potential energy surface of protonated formamide has not been reported in the literature so far. Hence, we have considered it of interest to carry out a systematic study, using high-level ab initio techniques, of this potential energy surface, which should provide useful information to rationalize some aspects of the unimolecular chemistry of the protonated derivatives of formamide and its two tautomers. This will also allow us to compare its behavior with that of a closely related system such as formamidine, recently studied by us.31
-
-
- -
-
Computational Details In order to have as reliable energetics as possible, the total energies of formamide (l), formamidic acid (2), (aminohydroxy)carbene (3), and of their protonated species and the transition states connecting them were obtained in the framework of G2 theory. G2 theory32is a composite procedure based in Moller-Plesset perturbation theory at second and fourth orders (MP2 and MP4) and quadratic configurationinteraction including single, double, and triple excitation (QCISD(T)) levels of theory. In the G2 procedure a total energy effectively of QCISD(T)l
0022-365419512099-13890$09.00/0 0 1995 American Chemical Society
*
J. Phys. Chem., Vol. 99, No. 38, 1995 13891
Protonated Formamide and Formamide-X+
K
0.989
1 . a
121.2
/H
Hp= 118.8
P 110.4 p ,
k
H 3
1
‘\\
;.H
1.273
0.974
TS12
TS13
TS23
Figure 1. MP2/6-31G*-optimized structures of formamide (l),formamidic acid (2), (aminohydr0xy)carbene (3) and the transition states of the corresponding isomerization reactions: bond lengths in angstroms and bond angles in degrees.
6-31 l+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 to yield ionization energies, atomization energies, proton affinities, and heats of formation in agreement with the experimental values within kO.1 eV. Nevertheless, this procedure becomes very expensive for the treatment of the metal ion complexes, in particular when they involve second-row monocations such as Na+, Mg+, or Al+. Therefore, the total energies of the different [C,N,H3,0,X]+ (X = Li, Na, Mg, and Al) complexes were obtained by using a variation of G2 theory, usually referred as G2(MP2)35 theory, where the basis-set-extension energy corrections are obtained at the second-order Moller-Plesset (MP2) level. It has been shown35that, in general, the average absolute deviation of G2(MP2) theory is 1.58 kcdmol, compared to 1.21 kcal/mol for the G2 theory. Furthermore, in a recent study of Li+, Na+, Mg+, and Al+ complexes of formamidine, we have shown that the G2(MP2) scheme yields relative energetics in agreement with the G2 formalism within k0.2 kcdmol. It should also be indicated that although, in the original G2 and G2(MP2) schemes, the zero point energy (ZPE) corrections are evaluated at the HF/6-31G* level, in our calculations the harmonic vibrational frequencies were obtained at the MP2/631G* level and the ZPE corrections were scaled by the empirical factor 0.93. No symmetry constraints were imposed in the geometry optimizations. These ab initio molecular orbital calculations were carried out by using the Gaussian 9036 and Gaussian 92 program packages.37 The nature of the bonding interactions of the different molecular ions included in this paper were investigated by means of a topological analysis of the electronic charge density, Q, and its Laplacian, VQ.As has been shown by Bader and c o - w ~ r k e r s , ~V~*e - ~identifies ~ regions of space where the electronic charge is locally depleted (VQ> 0) or built up (VQ < 0). The former situation is typically associated with interactions between closed-shell systems (ionic bonds, hydrogen
bonds, and van der Waals molecules), while the latter characterizes covalent bonds, where the electronic charge concentrates between the nuclei. We have also located the relevant bond critical points (bcp’s), i.e., points where the electronic charge density is minimum along the bond path and maximum in the other two directions, because the values of e and ve at these points offer quantitative information on the strength and nature of the bonding. Furthermore, the values of the negative curvatures (Ai,A2) allow us to define the ellipticity of the bond as E = 1 - A,/&. This topological analysis was carried out on the wave function correct to first order, to take explicitly into account electronic correlation effects. For this purpose we have employed the AIMPAC series of program^.^'
Results and Discussion Neutral Systems. In order to investigate the question of metal-mediated isomerizations between formamide (1) and its two tautomers (2 and 3), we have first calculated the three neutrals and the transition states between them, namely TS12, TS13, and TS23, at the G2 level of theory. Although the geometries of the first two species have been reported in the literature at the same or similar levels of accuracy, we have included them in Figure 1 to facilitate the discussion of the structural changes found upon proton and metal association. Total and relative energies are given in Table 1. Although for formamidic acid (2) the four possible conformers have been investigated, only the most stable s-cis form is included in both Table 1 and Figure 1. For tautomer 3, the s-trans form lies about 25 kcaVmol below the s-cis conformer. For this carbenic species we have also studied the corresponding triplet state. Our results (at the MP2/6-31G* level) show it to lie much higher in energy ( ~ 5 kcdmol) 4 than the singlet. Therefore, in what follows we have considered only the most stable singlet state form. The stability of the m3Co species (4), which would be the result of a hydrogen shift from carbon to nitrogen in formamide, was also studied. Similarly to what was found at the HF/4-31G level: this hydrogen shift is accompanied, at
Tortajada et al.
13892 J. Phys. Chem., Vol. 99, No. 38, 1995
TABLE 1: G2 and G2(MP2) Total Energies (hartrees), Relative Energies AE (kcdmol), and Dipole Moments p (Debyes) for Formamide (l),Formamidic Acid (2). (Aminohydroxykarbene (31, and the Transition States between Them compound
1
2
3
TS12
TS13
TS23
E(G2) E(G2MP2) A E" P
- 169.64652 - 169.64196 0.0 (0.0) 4.3 197.6 (197.8)b
- 169.62822 - 169.62383 11.5 (11.4) 1.2 209.1 (209.2)'
- 169.58666 - 169.58243 37.6 (37.4) 1.3 235.2 (235.2)d
- 169.56993 -169.56881 46.4 (45.9) 3.1
-169.52797 - 169.52728 72.5 (72.0) 2.8
- 169.5 1354 -169,50935 83.4 (83.2) 1.8
Oxygen protonation. Nitrogen a Values within parentheses correspond to G2(MP2) calculations. b Proton affinities of the neutrals in kcallmol. protonation. e Carbon protonation.
TABLE 2: Charge Densities (in dau3),Laplacian of the Charge Densities V p (in dad), and Ellipticities (e) Evaluated at the Corresponding Bond Critical Points (bcp's) of Some Bonds of the Complexes Included in This Study
c-0
N-C
1 1Li 1Na 1Mg 1Al 2 2Li' 2Na' 2Mg' 2Al' 3 3Li" 3Na" 3Mg" 3A1"
e
Ve
E
e
Ve
E
0.316 0.345 0.339 0.35 1 0.355 0.392 0.392 0.392 0.382 0.377 0.328 0.345 0.342 0.348 0.348
-0.879 -0.863 -0.869 -0.863 -0.853 -1.058 - 1.020 - 1.020 - 1.030 - 1.036 -0.569 -0.701 -0.684 -0.713 -0.694
0.098 0.163 0. I47 0.179 0.193 0.327 0.335 0.343 0.281 0.257 0.101 0.025 0.005 0.051 0.056
0.397 0.366 0.373 0.357 0.348 0.296 0.283 0.284 0.308 0.315 0.279 0.301 0.298 0.306 0.308
0.101 -0.03 1 -0.028 -0.065 -0.121 -0.309 -0.296 -0.305 -0.261 -0.248 -0.095 -0.171 -0.160 -0.169 -0.152
0.117 0.035 0.055 0.020 0.005 0.005 0.033 0.021 0.012 0.004 0.437 0.216 0.249 0.177 0.170
the MP2/6-31G* level, by a C-N bond fission. The result is a loosely bound dipole-dipole complex, between ammonia and carbon monoxide. For this reason we have not included this form in what follows. The bonding characteristics of the three neutrals (see Table 2) are consistent with the structural changes that take place in the tautomerization processes. It is interesting to note that the charge densities and the ellipticities at the C-N and C-0 bonds of 3 are quite high, showing the existence of an important delocalization of charge in the corresponding N-C-0 framework. It can be seen that, at the G2 level, formamidic acid (2) and (aminohydr0xy)carbene (3) are predicted to be 11.5 and 37.6 kcdmol less stable than formamide (l),in good agreement with previous estimation^.^^^'^^^ Also the corresponding isomerization barriers are not significantly different from previously reported values,"~'0~21 although 2.5-4.5 kcdmol lower. More importantly, it can be observed (see Table 1) that these relative values do not change significantly when obtained at the more economic G2(MP2) level, the greater difference, with respect to G2 values, being 0.5 kcal/mol. Since tautomer 3 has been generated recently in the gas-phase,'O we have considered it of interest to report its harmonic vibrational frequencies (see Table 3). Protonated Systems. We present in Figure 2 the [&,C,N,O]+ potential energy surface, which would correspond to the protonation of formamide and its tautomers. For the sake of clarity we have adopted the following convention: the different minima are identified by the number of the corresponding neutral followed by H if protonation takes place at the oxygen atom and by H' if the protonation occurs at nitrogen. A similar convention will be adopted later for metal monocation association; Le., the different minima will be identified by the number of the corresponding neutral followed by X, X', or X" if association takes place at the oxygen, nitrogen, or carbon atom, respectively. The different transition states (TS's) are named by indicating the two minima they connect (TSlH2H, TS3H3H', etc.). The transition states which correspond to the dissociation
0-x
N-X
e
Ve
E
0.026 0.021 0.036 0.046
0.175 0.124 0.224 0.171
0.027 0.057 0.068 0.081
0.028 0.022 0.034 0.047
0.149 0.107 0.158 0.093
0.023 0.027 0.029 0.024
e
Ve
E
0.038 0.028 0.042 0.053
0.357 0.220 0.370 0.363
0.047 0.058 0.032 0.020
0.015 0.012
0.099 0.071
0.853 0.267
TABLE 3: Harmonic Vibrational Frequencies (cm-l) for (Aminohydr0xy)carbene(3) 3805 3754 3532 1685 1473 1319 1213 1049 798 64 1 563 535
.
OH stretching NH stretching NH' stretching" NH2 scissors CN stretching CO stretching NH2 rocking OH bending CO stretching NH1 twisting OH bending out of plane OH bending out of plane 0-C-N bending NH2 wagging
+
+
" H' denotes the hydrogen atom cis with respect to the OH group. of a given minimum into products are identified by the symbol of the minimum followed by P (TSlHP, TSZHP, etc.). The G2 total energies have been summarized in Table 4, and the MP2/6-31G* optimized geometries are presented in Figure 3. Table 4 also contains the G2 energies of possible fragment products. As is well known, formamide is found to be an oxygen base in the gas-phase, the nitrogen protonated species being 15.5 k c d mol less stable. Its predicted 0-proton affinity (197.6 kcal/ mol) is in very good agreement with the experimental value (198.4 kcal/m01)~*and considerably lower than that of formamidine (225.3 k ~ d m o l ) . ~It' should be noted that the 1H species also corresponds to the protonation of formamidic acid (2) at the nitrogen atom (1H = 2H') and to the protonation of (aminohydr0xy)carbene (3) at the carbon atom (1H = 3H"). This implies that these two neutrals would behave as very strong nitrogen and carbon bases, respectively, in the gas-phase (see Table 1). Once more it may be seen that the relative values obtained at the G2(MP2) level are very close to the G2 ones, although the average differences are slightly larger than those found for the neutrals. Oxygen- and nitrogen-protonated formamide (1H and 1H') are connec' lwite high activation barrier (53.9 kcdmol),
J. Phys. Chem., Vol. 99,No. 38, 1995 13893
Protonated Formamide and Formamide-X+
j.6" OH+HCNd
'
,,."
II
1SO NH8 a ' -, ; 1m.o
' ,
, . ,I ,, I
' (
,
', I
'\
I
,
I
I
4
,
'
I
I
:
, 1
W(86.7)
,
HP+HCNH?
I
L
47.4
w*3.0) I
/
\ %
!
I
i 'j
!, I
* $8,
I
,
\
'
I %,
I ,
\ I
1n (0.0)
1H'(16.6)
I,'
'$IN&+
CO
t .(I
Figure 2. Schematic representation of the [&,C,N,O]+ potential energy surface: all values in kilocalories per mole.
TABLE 4: 6 2 and G2(MP2) Total Energies (in hartrees), and Relative Energies (in k d m o l ) for the [&,C,N,O]+ Complexes Included in This Study species E(G2) E[G2(MP2)] AE6 AHof,expa 1H 1H' 2H 3H 3H' TSlHlH' TSlH2H TSlH3H TSlH3H TSlHP TSlH'P TS2H3H TS2HP TS3H3H' TS3H'P HCNH2'
co
HzO NH3 NH4+ HCNH+ CNH2' HCO' COH'
-169.961 50 -169.937 39 -169.893 03 -169.856 82 -169.858 95 -169.875 65 -169.867 49 -169.815 00 -169.819 250 -169.727 47 -169.897 71 -169.794 67 -169.890 32 -169.791 02 -169.832 78 -94.104 00 -113.17749' -76.332 05' -56.458 65' -56.781 40' -93.553 88 -93.475 13 -113.401 10 -113.341 83
-169.957 -169.930 -169.888 -169.852 -169.855 -169.869 -169.861 -169.809 -169.813 -169.720 -169.895 -169.790 -169.886 -169.787 -169.825
19 87 81 98 23 80 65 22 37 69 84 91 08 43 79
-113.17540d -76.330 Old -56.45 71Sd -56.779 8Sd
0.0 (0.0) 15.5 (16.5) 43.0 (42.9) 65.7 (63.4) 64.4 (64.0) 53.9 (54.8) 59.0 (60.0) 91.9 (92.9) 89.3 (90.2) 146.9 (148.4) 40.0 (38.5) 104.7 (104.3) 55.7 (54.6) 107.0 (106.5) 80.8 (82.5)
123.0
+
-.
-
258'0 -26.5 -58.0 -11.0 151.0 226 265.0 197.3 230.0
a Experimental heats of formation (in kcavmol) when available. Values taken from ref 42. 6 Values within paentheses the G2(MF2) relative energies. - c Values taken from ref 32. Values taken from ref 35.
which indicates that both species may be experimentally accessible. This seems to be in agreement with the fact that is one of the most abundant products of the unimolecular decomposition of protonated f ~ r m a m i d e .Actually, ~ our PES
m+
shows that the nitrogen-protonated species (1H') would yield CO N&+, through an energy barrier of 24.5 kcaYmo1. Also interestingly our G2 calculations predict these products to be 1.6 kcaYmol above oxygen-protonated formamide (lH), in excellent agreement with experimental findings (1.5 kcaYmo1).42 The loss of OH from the global minimum (1H) to yield HCNH2+ has a quite high energy barrier (146.9 kcaumol), in agreement with the fact that no loss of OH is experimentally ~bserved.~ On the contrary, the barrier for the 1H 2H isomerization is much lower and only slightly higher than the 1H 1H' one. Species (2H) is a precursor for H20 loss, yielding HCNH+, also experimentally observed. It must be noted however that this fragmentation takes place through a transition state (TS2HP) which lies 12.7 kcaYmol above 2H. The energy barriers between oxygen- and nitrogen-protonated formamide (1H and 1H') and the protonated forms of 3 (3H and 3H') are much higher. Also high are the barriers which connect the 3H and 3H' species to each other and to species 2H or to the possible products. Hence, we can reasonably conclude that they are also experimentally accessible. The oxygen-protonated species (3H) can lead, as can 2H, to the loss of water, yielding CNH2+ (which is 49.4 kcaYmol less stable than the HCNH+ isomer). We have located, at the MP2/6-31G* level a TS corresponding to this process. However this TS was found to lie below the products when its energy was evaluated at higher levels of accuracy. Hence we conclude that association of CNH2+ to H20 to yield the 3H species takes Place without an activation barrier. Similarly to lH', the nitrogen-protonated form (3H') would yield N&+ CO, through an energy barrier of 28.6 kcdmol. To confirm without ambiguity that both the 1 H and 3 H species would yield CO as final products, we have performed
+
m++
Tortajada et al.
13894 J. Phys. Chem., Vol. 99, No. 38. 1995
wq+
I+
I
110.7
vi
li 2H
1H
‘
I+ ,.i.im 1dB7
H
(SI 11W
l.m
3H’
.
TS1 H3H
1910 116.1
TSlHIH’
’ 1.-
115.2
‘*
)f’l.aal
‘H TS1 HP
TS2H3H
1
I+
TS1H‘P
TS1H’3H’
TS3H3H’
Figure 3. MP2/6-3 1G*-optimized structures of the stationary points of the [&,C,N,O]+ potential energy surface: bond lengths in angstroms and bond angles in degrees.
an intrinsic reaction coordinate (IRC) analysis43following the adequate direction of the corresponding transition vector (that associated to TSlH’P and TS3H’P, respectively). For both cases the resulting structure corresponded to a complex between N&+ and CO as a result of a proton transfet from the HCO+ (or COH+) moiety to the NH3 one. This is not surprising if one takes into account the enormous difference between the proton affinities of ammonia and carbon monoxide either as a carbon or as an oxygen base. Closely related to this is the problem raised by the observed loss of ammonia from metastable ions. As shown in ref 9 by means of dual-cell Fourier transform mass proton-transfer experiments and high-resolution labeling results, the loss of ammonia does not occur by a mechanism which involves H/D exchanges. Consequently, only 1H’ and 3H’ minima can be taken as precursors for ammonia loss with the formation of the HCO+ (or COH+) species. However, the 1H’ species is much
+
lower in energy than the final NH3 HCO+ products, and as we have mentioned above, it leads through TSlH’P to the most CO. Lin et al.9 assume that ammonia stable products N&+ is formed from all 0-protonated molecules which rearrange via TSlHlH’ to give HCO+ NH3 rather than the thermodynamiCO. This implies necessarily that cally more stable N&+ the products (HCO+ NH3) must be lower in energy than TSlHlH’, in contrast with our results which show that, even at a very high level of accuracy, TSlHlH’ lies about 10 kcal/ mol below the products. Furthermore, when an intrinsic reaction coordinate analysis following both directions of the transition vector is performed, the only equilibrium structures found are 1H and 1H’. More significantly, all along the reaction coordinate, carbon and nitrogen atoms remain tightly bound. This also indicates that the argument given in ref 9 that the high value of the imaginary frequency (typical of a H transfer
+
+
+ +
J. Phys. Chem., Vol. 99, No. 38, 1995 13895
Protonated Formamide and Formamide-X+
Q
m
’k
m
k
0
A 40.00
W
h 30.00 Q,
cl
W
X
20.00
?
i
3
Q 10.00
, J ,
I
I
,
I
I
I
,
I
5.0
I
I
10.0
distance ( A )
C-N -j
d
-I
1
\50.00 4
Q
0
3 40.00 v
I
I
Q,
>
-4
3
Q 10.00
0.001 0.0
,
I
,
1
,
5.0
C-N
I
I
1
10.0
,
I
distance ( A )
Figure 4. Potential energy curves corresponding to the C-N bond fission process of (a) N-protonated formamide protonated (aminohydroxy)carbene (3H’).
(1H’)and (b) N-
process) implies a large momentum transfer from hydrogen to nitrogen which would break the C-N bond is not correct. Since, as mentioned above, only the 1H’ and 3H‘ species can be precursors for NH3 loss, we have investigated whether a C-N bond fission, in both cases, can yield NH3 HCO+ (or COH+). For this purpose we have optimized the structure of the 1H’ and 3H’ systems for different values of the C-N bond length, starting from the equilibrium value up to a C-N distance of 10 A, which can be taken as a long enough distance to consider that the C-N linkage is completely broken. The results obtained have been represented in Figure 4, where the different optimized conformations for each C-N distance have been superposed to the potential energy curve. It is evident that, in both cases, a C-N bond fission would eventually lead to a iondipole complex between NH3 and HCO+ (or COH+). However, it must be taken into account that for a C-N distance of about 2.5 8, TSlH’P and TS3H’P are found, which would lead to N&+ CO, as we have indicated above. Hence, we might conclude that ammonia will be formed only if the amount of energy deposited in the precursor is high enough to produce a
+
+
I
/’p.
\/
1’
Job/” DPI
fast C-N bond fission. On the contrary, if TSlH’P or TS3H’P exist for several rotational periods, they will rearrange to yield only the most stable W+ CO products. Metal Cation Association. Metal cation association of formamide (1) takes place at the oxygen atom, but while protonation at the amino nitrogen leads to a local minimum of the PES, metal cation association at the amino nitrogen evolves, without an activation barrier, to yield the oxygen-attached species. This reflects the fact that the interaction between formamide and the metal monocations considered is essentially electrostatic,the dominant term being the ion-dipole interaction. To enhance this interaction, the metal monocation locates in the direction of the overall dipole moment of the neutral and only the oxygen-attached species is found to be stable. Accordingly, the charge densities at the 0-X bond critical points are typically small and the Laplacians are positive (see Table 2). For formamidic acid (2) three different conformers have been found to be local minima of the PES (see Figure 5). The most stable corresponds to complex 2X‘, where the metal cation may interact with both basic centers simultaneously. An analysis of the corresponding charge densities clearly shows the existence of a bcp in both N-X and 0-X linkages for the case of Li+ and Na+. However, in Mg+ and Al+ complexes the metal is attached only to the nitrogen atom. This is a peculiarity of Mg+ and Al+,which, contrarily to alkali cations, have low-lying empty 3p-orbitals, and they are more polarizable. The accessibility of these 3p-orbitals favors the formation of dative bonds from the neutral to the cation, which gives a certain covalent character to the corresponding linkage. Since oxygen is more electronegative than nitrogen, the dative bond involves the lone pairs of the latter rather than those of the former. A similar behavior has been observed for other Mg+- and Al+containing in particular for those involving formamidine.31 For (aminohydroxy)carbene (3), three conformers are also stable: two of them (3X” and 3X”a) correspond to carbon attachment, while the less stable (3X) corresponds to oxygen attachment. The most stable conformer corresponds always to the 3X” complex. The optimized structures of the most stable conformations of these metal monocation complexes are presented in Figure 6. Those of the remaining local minima are available from the authors upon request. The corresponding G2(MP2) total energies are given in Table 5 , together with the binding energies. Some points should be singled out for comment. Although formamide is a weaker base than formamidine as far as protonation is concemed, both bases have very similar Li’, Na+, Mg+, and Al+ binding energies. In fact the dipole moments of both neutrals are similarly high, and accordingly, both systems yield strong ion-dipole interactions. As was found for formamidine, the binding energies of Mg+ and Al+ are greater than that of Li+. This is due, on one hand, to the enhanced interaction between the neutral and Al+ or Mg+, through the
+
0.001 0.0
n60.00
v 1.
Figure 5. Stable equilibrium conformations for the metal cationization of formamidic acid (2) and aminohydroxy carbene (3).
Q,
.rl
1.
vx
“\/
Tortajada et al.
13896 J. Phys. Chem., Vol. 99, No. 38, 1995
q+
N7 +
/,,e
d) 1.014p21.2
165.0
k
H
1u
1Na
l+
I *
k
2Na'
k
w
I+
I+ 1.m
1.311)
I
1.314
k 3Na.
3un
3Mgn
3AI"
Figure 6. MP2/6-3 1G*-optimized structures of the global minima corresponding to the cationization of formamide and its tautomers by Li+, Na+, Mg+ and AIC: bond lengths in angstroms and bond angles in degrees.
TABLE 5: G2(MP2) Total Energies for the CNO&X+ Complexes Included in This Study Li+ 1x 2x 2X' 3x 3X" TSlX2X TSlX3X TS2X2X' TS2X3X TS3X3X
xco
- 176.952 52 (46.9) -176.901 44 (26.2) -176.926 20 (41.7) - 176.857 29 (24.5) -176.88482 (41.8) - 176.869 28 -176.815 49 -176.893 22 -176.780 19 -176.848 72 -120.435 58 (15.3)
Na' -331.359 02 (33.1) -331.318 96 (19.3) -331.334 84 (29.3) -331.270 17 (14.8) -331.292 68 (28.9) -33 1.278 36 -331.230 52 -331.305 00 -331.208 41 -331.259 37 -274.854 86 (9.5)
Mg+ -369.083 43 (48.6) -369.027 26 (24.6) -369.061 10 (45.8) -368.984 45 (24.2) -369.023 16 (48.0) -368.998 51 -368.949 08 -369.016 43 -368.923 76 -368.974 83 -312.561 45 (13.8)
Al+ -41 1.431 33 (47.6) -411.371 94 (21.5) -41 1.401 39 (40.0) -411.329 23 (20.7) -411.363 01 (41.9) -41 1.344 49 -41 1.294 88 -41 1.365 00 -411.250 96 -411.322 59 -41 1.322 59 (7.6)
a Values within parentheses correspond to the binding energies in kcaymol. We include also the values corresponding to the XCO+ complexes for the sake of completeness.
dative bond from the lone pair of the former to the empty 3porbitals of the latter,31and, on the other hand, to the fact that Al+ and Mgf are much more polarizable than Li+. Both effects are mirrored in a greater charge density at the 0-X bcp's (see Table 2). Both formamidic acid (2) and (aminohydr0xy)carbene (3), contrarily to what has been found for proton association, present metal-binding energies smaller than those of formamide. In the case of formamidic acid it must be noted that the global minimum corresponds to the association to the s-tram conformer of the neutral rather than to the most stable s-cis conformer. In other words, upon metal association to the nitrogen atom, the hydroxyl group rotates to avoid the repulsion between the incoming monocation and the positively charged hydroxylic hydrogen atom and to favor the formation of the bridged
structure 2X. However, this rotation has an energetic cost in the neutral of the order of 7 kcal/mol, which explains the lower formamidic acid binding energies. Complexation to (aminohydroxy)carbene (3) involves the most stable conformer, which presents a dipole moment much smaller than that of formamide and accordingly smaller binding energies. It is interesting to notice that complex 3 X a , which corresponds to the association to the other conformer of 3 (which has a greater dipole moment), is not more stable. This is so because, although in this case the ion-dipole interaction is much larger, the energy cost associated with the OH rotation is also very large (about 25 kcal/mol at the MP2/6-31G* level), due to the loss of conjugation mentioned in previous sections and to a significant increase in the H.**H repulsion. The most important consequence is that, while for other systems, such as
Protonated Formamide and Formamide-X+
J. Phys. Chem., Vol. 99,No. 38, 1995 13897
A
Tszx2a(20.7,18.7,28.0,22.8)
zx
d zw
(52.3,50.6,!53.3,54.6)
(Q1.6,7Q.3,88.2,Q4.4) Nt$ + xco*
1x
(22.6,20.Q, 30.3,25.3)
TslXsX
w 9x
Figure 7. Isomerization processes between the local minima of the [X,H3,C,N,O]+ potential energy surface. The values within parentheses correspond to the barriers for X = Li, Na, Mg, and Al, respectively. All values are in kilocalories per mole and refer to the most stable minimum which correlates to each arrow.
acetaldehyde?’ aziridine? and f~rmamidine,~’ metal cation association implies a sizable stabilization of their carbenic tautomers, for formamide the (aminohydr0xy)carbene-formamide energy gap increases upon metal cation association. In Figure 7 we present a schematic representation of the PES’S of these metal-containing complexes. The most important conclusion of this study is that none of the four metal monocations investigated has a catalytic effect on the isomerization of formamide. Figure 7 shows, for instance, that isomerizations of the 1X species to yield 2X complexes imply energy barriers which, on average, are 5 kcaYmol higher than that between the neutral forms of formamide and formamidic acid. Similarly, the 1X 3X and 2X 3X isomerization barriers are on average 10 and 20 kcdmol higher than those for the 1 3 and 2 3 isomerizations, respectively. This can be rationalized in terms of changes in the ion-dipole interactions. The hydrogen shift involved in the 1 2 and 1 3 isomerization processes changes the direction and the magnitude of the dipole moment of the system (see Table 1). As a consequence, the interactions of TS12 and TS13 with the metal cations to yield TS1 X 2X and TS1 X 3X are weaker than those for formamide, and the corresponding isomerization barriers are higher. The 2X 3X isomerization barrier is also higher than the 2 3 one, in spite of the fact that the dipole moment of TS23 is greater than those of the corresponding equilibrium structures. This is so because (i) in TS2 X 3X the metal cation, which remains necessarily attached to the oxygen atom, is not in the direction of the dipole moment of the TS23 moiety and (ii) there is a repulsive interaction between the metal cation and the N-H hydrogen which is not present in TS23. As shown in Figure 7, isomerization of the 1X complexes leads to the oxygen-attached complexes of formamidic acid (2X) and (aminohydr0xy)carbene (3X), which are connected to the global minima (2X and 3X”), respectively, by much lower
-
-
-
-
- -
-
-
energy barriers, which involve essentially the rotation of the HOX group. It must be emphasized that a hydrogen shift from oxygen to nitrogen in the 3X“ species would yield the NH3C(X)O species, which would formally correspond to complexes between the unstable NH3CO species (4) and the metal cation. This 1,3 hydrogen shift implies energy barriers of about 46 kcdmol (see Figure 7) and results in a C-N bond fission similar to that found for the corresponding neutral. Hence we can conclude that two alternative reaction paths are open for the 3X“ species: the isomerization path through TS3 X 3X” to yield the 3X species and the dissociation into N H 3 XCO+ through the TS3X’T transition states. In this respect, it should be mentioned that the latter processes are exothermic for Li+ and Na+ by 5 and 12.1 kcdmol, respectively, while they are endothermic by 2.8 kcdmol for Mg+ and Al+. These results reflect the very different binding energies of these metal monocations to carbon monoxide (see Table 5).
+
Conclusions The isomerization barriers from formamide to formamidic acid and to (aminohydr0xy)carbene have been evaluated at the G2 level of theory. The values obtained are similar to those previously reported in the literature but slightly smaller. More importantly, these processes are not catalyzed by association to Li+, Na+, Mg+, or Al+ in the gas-phase because cationization by these metal ions is essentially electrostatic and the hydrogentransfer processes leading from formamide (1) to formamidic acid (2) or to (aminohydr0xy)carbene (3) involve changes in the direction and magnitude of the dipole moment which result in weaker ion-dipole interactions. A complete survey of the [&,C,N,O]+ PES shows that formamide is an oxygen base which can isomerize to the less
13898 J. Phys. Chem., Vol. 99, No. 38, 1995 stable nitrogen-protonated species through a barrier of 53.9 kcall mol. The nitrogen-protonated species is a precursor for the formation of N&+, which is the main product observed in the unimolecular decomposition of protonated formamide. The oxygen-protonated formamidic acid, which is connected to 0-protonated formamide through an isomerization barrier of 59.0 kcaymol, is the precursor for the water loss reaction, also observed as a product of the decomposition of metastable ions. The barriers of isomerization between the protonated forms of formamide and those of (aminohydr0xy)carbene are much higher. We have provided evidence that the mechanism proposed in the literature to explain the formation of HCO+ from the TS connecting the two protonated forms of formamide (1H and 1H’) is not possible. Production of ammonia can only take place from the N-protonated forms of formamide (1H’) and (aminohydr0xy)carbene (3H’). An investigation of the corresponding potential energy curves shows that a C-N bond fission of both the 1H’ and 3H’ species would lead to an iondipole complex between ammonia and HCO+ (or COH+) only if this process is fast enough to compete with the production of N&+ CO, which takes place through the TSlHT and TS3HT transition states.
+
Although formamide is a weaker base than formamidine as far as proton association is concerned, its gas-phase basicity toward Li+, Na+, Mg+, and Al+ is similar. As it was found for other bases,32,44,46,47 formamide presents enhanced basicities with respect to Mg+ and Al+, due to the fact that for these two cations the interaction, although essentially electrostatic, has a certain covalent character, through a dative bond from the lone pairs of the base to the empty 3p-orbitals of the metal. Contrarily to what has been found for other bases such as a~etaldehyde,~’aziridine,6 or f~rmamidine,~’ cationization by metal monocations in the gas-phase does not stabilize (aminohydroxy)carbene (3) with respect to formamide. This is due to the significant decrease of the polarity of the system on going from formamide to the carbene.
Acknowledgment. This work has been partially supported by DGICYT Project No. PB93-0289-C02 and by Accidn Integrada Hispano-Francesa No. 102B. References and Notes (1) (2) (3) 8334. (4)
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