Interaction of Formic Acid with Nitrogen: Stabilization of the Higher

Sep 10, 2010 - Sergey V. Ryazantsev , Vladimir I. Feldman , and Leonid Khriachtchev. Journal of the American Chemical Society 2017 139 (28), 9551-9557...
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Interaction of Formic Acid with Nitrogen: Stabilization of the Higher-Energy Conformer Kseniya Marushkevich,* Markku Ra¨sa¨nen, and Leonid Khriachtchev* Department of Chemistry, P.O. Box 55, FIN-00014 UniVersity of Helsinki, Finland ReceiVed: June 2, 2010; ReVised Manuscript ReceiVed: August 13, 2010

Conformational change is an important concept in chemistry and physics. In the present work, we study conformations of formic acid (HCOOH, FA) and report the preparation and identification of the complex of the higher-energy conformer cis-FA with N2 in an argon matrix. The cis-FA · · · N2 complex was synthesized by combining annealing and vibrational excitation of the ground-state trans-FA in a FA/N2/Ar matrix. The assignment is based on IR spectroscopic measurements and ab initio calculations. The cis-FA · · · N2 complex decay in an argon matrix is much slower compared with the cis-FA monomer. In agreement with the experimental observations, the calculations predict a substantial increase in the stabilization barrier for the cis-FA · · · N2 complex compared with the uncomplexed cis-FA monomer. A number of solvation effects in an argon matrix are computationally estimated and discussed. The present results on the cis-FA · · · N2 complex show that intermolecular interaction can stabilize intrinsically unstable conformers, as previously found for some other cis-FA complexes. Introduction Understanding of the role of conformational isomerism is one of the current challenges in physical and biological chemistry.1-3 Quantum tunneling of hydrogen atom is an important mechanism of the isomerization around the C-O bonds in carboxylic acids and alcohols at low temperatures.4 For formic acid (HCOOH, FA), the simplest carboxylic acid, two conformers are possible, the cis form being less stable than the trans form by ca. 1365 cm-1.5 Upon vibrational excitation in rare-gas matrices, the ground state trans-FA conformer changes to the higher-energy cis-FA conformer, and cis-FA decays to trans-FA in the dark via tunneling of the hydrogen atom through the C-O torsional barrier.6,7 Similar conformational processes have been found for a number of other carboxylic acids in cryogenic matrices.8,9 Hydrogen tunneling after UV-induced isomerization has also been reported for hydroquinone, substituted hydroquinone, and 2-chlorobenzoic acid in argon and xenon matrices.10-12 Hydrogen atom tunneling has been studied for intermolecular complexes containing hydrogen bonds, which causes a significant change in the asymmetric potential surfaces compared with the monomeric forms. For example, the intermolecular O · · · H bonding stabilizes cis-FA upon complexation with a water molecule or oxygen atom so that the hydrogen tunneling is completely suppressed.13,14 Intermolecular interactions can stabilize the higher-energy conformer even if the tunneling hydrogen atom does not participate in the bonding directly.15,16 In the present work, we report the first complex of the higherenergy cis-FA conformer with a nitrogen molecule in solid argon. The earlier experiments and calculations on the FA-nitrogen complexes have been limited to the low-energy trans conformer.17 The identification of the new complex is done using ab initio calculations and infrared spectroscopy. The decay of the cis-FA · · · N2 complex in an argon matrix is found to be * Corresponding authors. E-mails: (K.M.) kseniya.marushkevich@ helsinki.fi, (L.K.) [email protected].

much slower compared with the cis-FA monomer. Solvation effects in an argon matrix are computationally estimated and discussed. Computational Details and Results Computational Details. The calculations were performed using the Gaussian 03 package.18 The equilibrium geometries, reaction barriers, and vibrational frequencies of the trans- and cis-FA monomers and the complexes with nitrogen and water were calculated using the second-order Møller-Plesset perturbation method [MP2 ) full/6-311++G(2d,2p)] in the harmonic approximation with the default convergence criteria. The counterpoise method was used to account for the basis set superposition error (BSSE),19 and zero-point energies (ZPEs) were accounted for. The spectral shifts for the complexes were calculated as a difference between the complex and monomer vibrational frequencies. The tunneling barriers of the monomers and complexes were calculated by scanning the potential energy surface (PES) along the torsion coordinate. The final tunneling system for the cisFA · · · N2 complex was a non-interacting trans-FA + N2 pair. Upon the scan, the coordinates of all the atoms (except the tunneling H atom) were fixed, which was based on the adiabatic approximation for tunneling. We did the PES scans with and without BSSE corrections. ZPE corrections were not included in this estimate because ZPE cannot be found for non-optimized systems. The solvation energies of the trans- and cis-FA complexes with nitrogen and the cis-to-trans barriers in solid argon and xenon were estimated within the frame of the polarized continuum model (PCM) at the MP2(full)/6-311++G(2d,2p) level using the structures obtained in vacuum. Computational Results. Two planar trans-FA · · · N2 complexes (structures I and II in Figure 1) are in agreement with the previous results obtained by Lundell et al.17 Complex I with an O-H · · · N intermolecular bond (2.29 Å) has an interaction energy of -4.39 kJ/mol, and complex II bonded to the nitrogen molecule through the O atom in the OH group is 2.7 kJ/mol higher in energy. The interaction energies obtained by Lundell

10.1021/jp105044r  2010 American Chemical Society Published on Web 09/10/2010

Interaction of Formic Acid with Nitrogen

Figure 1. (a) trans-HCOOH · · · N2 and (b) cis-HCOOH · · · N2 complexes. The interaction energies in kJ/mol are given in parentheses (BSSE and ZPE corrected). Bond lengths are in Å.

et al. are slightly different, -6.07 and -2.12 kJ/mol for structures I and II. The inclusion of additional diffuse and polarization functions to the basis set presumably improves calculation of the interaction energy compared with the MP2/ 6-31G(d) basis set used by Lundell et al.17 Two planar complexes of the higher-energy cis-FA conformer with nitrogen were found on the PES (Figure 1). A more stable complex (structure III) has an O-H · · · N intermolecular bond (2.29 Å) and interaction energy of -4.79 kJ/mol. The second cis-FA · · · N2 complex (structure IV) has a C-H · · · N bond length of 2.84 Å and interaction energy of -1.53 kJ/mol. The structural parameters of complexed FA molecules are slightly different from the monomer, which is typical for weak intermolecular interactions. The largest spectral shifts correlate with the sites of complexation. For complexes I and III, the OH stretching (νOH) and torsional (τCOH) modes are the most sensitive to the presence of the nitrogen molecule (Tables 1 and 2). The largest shift occurs for the τCOH mode of structure III (+70.2 cm-1). For complexes II and IV, the largest shifts are obtained for the C-H stretching mode. The spectra for the deuterated HCOOD · · · N2 complexes have also been calculated (Tables 3 and 4). The conformational conversion occurs via the torsional motion of the OH group.7-9 The cis-to-trans torsional barriers of the FA monomer and FA · · · N2 complex (structure III) are shown in Figure 2. The transition states of FA monomer and FA · · · N2 are found to be similar in energy and dipole moment. The calculated barrier for cis-FA · · · N2 (without BSSE corrections 4013 cm-1, with BSSE corrections 3715 cm-1) is significantly higher than that for the FA monomer (3233 cm-1). The torsional barrier was also scanned for the FA-water complex identified previously in ref 13 and found to be 6758 cm-1. The solvation energies of the FA monomer, FA · · · N2 complexes, and transition states of the monomer and complexes in an argon matrix estimate the solvation effect on the tunneling barriers. The cis-to-trans barrier for monomeric FA increases in an argon matrix up to 3309 cm-1; i.e., by 76 cm-1 (∼2%). In contrast, for the FA-nitrogen complex III, the theory indicates a solvation-induced decrease of the torsion barrier to 3828 cm-1 (i.e., by 185 cm-1, ∼5%) when compared with the results in vacuum. In these estimates, BSSE corrections were not applied. Experimental Details and Results Experimental Details. The gaseous samples were prepared by mixing formic acid (HCOOH, Kebo Lab, 99%) or HCOOD

J. Phys. Chem. A, Vol. 114, No. 39, 2010 10585 (IT Isotop, 95-98%) with nitrogen and argon (AGA, 99.9999%). The matrices were deposited onto a CsI substrate at 9 K for HCOOH and at 4 K for HCOOD in closed-cycle helium cryostats (APD, DE 202A and Sumitomo Heavy Industries, respectively). The IR absorption spectra in the 4000-400 cm-1 spectral range were measured with Nicolet SX-60 FTIR spectrometers using resolution 1.0 cm-1 and coadding 100 or 500 interferograms. Conformational changes were promoted by an optical parametric oscillator (OPO Sunlite, Continuum with an IR extension) providing tunable IR light with pulse duration of 5 ns, spectral line width of 0.1 cm-1, and a repetition rate of 10 Hz. A Burleigh WA-4500 wavemeter measured the OPO signal frequency, providing an absolute accuracy better than 1 cm-1. During the kinetic measurements, a long-pass filter transmitting below 1800 cm-1 was installed between the Globar source and the matrix to eliminate broadband light-induced conformational conversion. The tunneling decay was studied by integrating the intensity of the COH-CO deformation bands. Experimental Results. Figure 3a (trace 1) presents the spectrum of a HCOOH/Ar (1/1000) matrix in the deformation spectral region. The bands of trans-HCOOH monomer (1103 cm-1) and two trans-trans dimers, tt1 (1225 cm-1) and tt2 (1180 and 1130 cm-1), are observed after matrix deposition.6,15,16,20 Weak absorptions of cis-HCOOH are seen at 1243 and 1249 cm-1.6 Annealing of the matrix at 28 K increases the amounts of the dimers (Figure 3a, trace 2). Vibrational excitation of trans-FA enhances the concentration of cis-FA. In a HCOOH/Ar matrix, annealing during irradiation leads to cis-FA monomer and a trans-cis dimer tc1 (1259 cm-1, see trace 3 in Figure 3a).16 Two trans-FA · · · N2 complexes in an argon matrix were previously reported by Lundell et al.17 Both structures (I, II) are found in our experiments after deposition of HCOOH/N2/Ar (1/3/1000) matrixes, characterized by the COH-CO deformation bands at 1114.3 and 1108.7 cm-1, respectively (Figure 3b, trace 4). Table 1 presents the experimental frequencies of the species in question. Annealing at 27 K increases the amount of the trans-FA · · · N2 complexes as seen in Figure 3b (trace 5). For matrices with nitrogen (HCOOH/N2/Ar ) 1/3/1000), annealing during irradiation at 4185 cm-1 (νOH + τCOH) produces a set of new bands (see Figure 3b, trace 6). These new bands at 3595.6, 1801.7, 1262.1, and 559.2 cm-1 are assigned to the νOH, νCdO, COH-COdef, and τCOH modes of complex III. The experimental frequencies of the cis-FA species are collected in Table 2. We performed similar experiments for deuterated formic acid (HCOOD) in an argon matrix with excitation at 4382 cm-1 (νOD + νCdO) and annealing at 30 K. The results with HCOOD are similar to those with HCOOH (see Figure 3c); however, the amount of cis-FA species is larger in experiments with deuterated FA due to the efficient suppression of back-tunneling.7,21 Tables 3 and 4 show the experimental frequencies for the trans and cis species obtained in the case of HCOOD. A more precise look at the result of the excitation at 4185 cm-1 upon annealing at 27 K of a HCOOH/N2/Ar matrix is provided by Figure 4a. The excitation converts trans-FA to cis-FA, and annealing leads to the formation of cis-FA · · · N2 (1262.1 cm-1). The spectra in Figure 4a are measured immediately after annealing and after 22 min in the dark at 9 K. It is seen that the cis-FA species decay with time; however, the decay rates are different. Figure 4b compares the decay rates of cis-FA · · · N2 and cis-FA obtained at 9 K. The lifetime of

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TABLE 1: Computational and Experimental Frequencies (cm-1) for the Characteristic Modes of trans-HCOOH Monomer and trans-HCOOH · · · N2 Complexesa,b mode

trans-FA, exptl

trans-FA, calcd

I, exptlc

I, calcd

shift I, exptl

shift I, calcd

II, exptlc

II, calcd

shift II, exptl

shift II, calcd

νOH νCdO COH-COdef τCOH

3550.5 1767.2 1103.4 635.4

3784.7 1788.4 1123.0 676.1

3531.9 1764.9 1114.6 679.8

3758.1 1785.0 1137.1 725.1

-18.6 -2.3 +11.2 +44.4

-26.6 -3.4 +14.1 +49.0

3538.1 1761.7 1108.7 638.3

3782.6 1787.0 1120.2 677.1

-12.4 -5.5 +5.3 +2.9

-2.1 -1.4 -2.8 +1.0

Shift is the difference between the complex and monomer frequencies. b I and II refer to the trans-FA · · · N2 complex structures I and II in Figure 1. c These data agree with ref 17. The computational data are in qualitative agreement. a

TABLE 2: Computational and Experimental Frequencies (cm-1) for the Characteristic Modes of cis-HCOOH Monomer and cis-HCOOH · · · N2 Complexesa,b mode

cis-FA, exptl

cis-FA, calcd

III, exptl

III, calcd

shift III, exptl

shift III, calcd

shift IV,c calcd

νOH νCdO COH-COdef τCOH

3615.9 1806.9 1248.9 505.3

3851.5 1829.6 1287.7 536.8

3595.6 1801.7 1262.1 559.2

3829.4 1825.9 1311.8 607.0

-20.3 -5.2 +13.2 +53.9

-22.1 -3.7 +24.1 +70.2

-0.9 -3.7 -1.0 +0.9

a Shift is the difference between complex and monomer frequencies. b III and IV refer to the cis-FA · · · N2 complex structures III and IV in Figure 1. c Complex IV is not observed in the experiments.

TABLE 3: Computational and Experimental Frequencies (cm-1) for the Characteristic Modes of trans-HCOOD Monomer and trans-HCOOD · · · N2 Complexesa,b mode

trans-FA, exptl trans-FA, calcd I, exptlc I, calcd shift I, exptl shift I, calcd II, exptlc II, calcd shift II, exptl shift II, calcd

νOD νCdO νC-O τCOD

2618.0 1767.2 1181.7 506.7

2752.0 1781.7 1195.3 531.5

2608.0 1764.8 1188.4 533.5

2732.8 1778.1 1204.0 560.4

-10.0 -2.4 +6.7 +26.8

-19.3 -3.6 +8.7 +28.9

2605.2 1762.2

2750.5 1780.2 1191.4 531.9

-12.8 -5.0

-1.6 -1.5 -3.9 +0.4

a Shift is the difference between the complex and monomer frequencies. b I and II refer to the trans-FA · · · N2 complex structures I and II in Figure 1. c These data agree with ref 17. The computational data are in qualitative agreement.

TABLE 4: Computational and Experimental Frequencies (cm-1) for the Characteristic Modes of cis-HCOOD Monomer and cis-HCOOD · · · N2 Complexesa,b mode

cis-FA, exptl

cis-FA, calcd

III, exptl

III, calcd

shift III, exptl

shift III, calcd

shift IV,c calcd

νOD νCdO νC-O τCOD

2668.7 1799.3d 1164.0

2803.9 1822.0 1177.5 403.7

2656.1 1794.6 1168.4 1172.0

2787.8 1817.5 1185.2 449.6

-12.6 -4.7 +4.4e +8.0

-16.1 -4.5 +7.7 +45.9

-0.7 -3.8 -3.5 +1.7

a Shift is the difference between the complex and monomer frequencies. b III and IV refer to the cis-FA · · · N2 complex structures III and IV in Figure 1. c Complex IV is not identified in the experiments. d The strongest band of Fermi resonance.21 e This band may belong to complex IV.

cis-FA · · · N2 is ca. 48 min, which is ca. 6.5 times longer than that of the cis-FA monomer (7.3 min). Discussion Spectral Assignment. Our computational and experimental results on the trans-FA · · · N2 complexes agree with the previous work by Lundell et al.17 Annealing at 27 K of an argon matrix containing FA and N2 enhances the trans-FA · · · N2 amounts (structures I and II). According to Lundell et al.,17 the trans-FA · · · N2 complex II reorganizes to the more stable structure I at 32 K in an argon matrix, and our annealing experiments at temperatures up to 35 K are in agreement with those observations. The preparation of the cis-FA · · · N2 complex uses annealing combined with vibrational excitation of trans-FA in a matrix containing nitrogen. The vibrational excitation enhances the amount of cis-FA, and annealing leads to the formation of the cis-FA · · · N2 complex. This strategy was previously used by us for the preparation of the cis-FA · · · H2O complex.13 The bands connected to the cis-FA · · · N2 complex appear in the spectra only when the matrix contains nitrogen and when vibrational

Figure 2. Calculated torsional barriers for the cis-to-trans conversion of the cis-FA monomer (squares), cis-FA · · · N2 complex III (stars), and the complex of cis-FA with water (cis-FAW, triangles). The data are presented without BSSE corrections.

excitation of trans-FA is performed during (or in the case of HCOOD, prior to) annealing. The amount of the cis-FA · · · N2 complex obtained with HCOOH is smaller compared with the

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Figure 4. Hydrogen atom tunneling in cis-FA monomer and cis-FA · · · N2 complex III. (a) Spectra in the COH-CO deformation region recorded at t ) 0 and 22 min at 9 K. The bands at 1249 and 1243 cm-1 belong to the cis-FA monomer in two matrix sites, and the band at 1262 cm-1 belongs to the cis-FA · · · N2 complex (structure III). (b) Relative concentration of cis-FA monomer and cis-FA · · · N2 complex as a function of time at 9 K. The lines show single exponential fits.

Figure 3. FTIR spectra in the COH-CO deformation region. (a) HCOOH/Ar ) 1/1000: (1) after deposition, (2) after annealing at 28 K, and (3) after irradiation at 3550 cm-1 (νOH) and annealing at 28 K. The band at 1173 cm-1 is probably the second site of the tt2 dimer. (b) HCOOH/N2/Ar ) 1/3/1000: (4) after deposition, (5) after annealing at 27 K, and (6) after irradiation at 4183 cm-1 (νOH + τCOH) and annealing at 27 K. I, II, and III mark bands of structures I, II and III shown in Figure 1. The spectra in panels a and b were measured at 9 K. (c) HCOOD in an argon matrix: (7) HCOOD/Ar ) 1/1000 after irradiation at 4382 cm-1 (νOD + νCdO) and annealing at 30 K; (8) HCOOD/N2/Ar ) 1/3/1000 after irradiation at 4382 cm-1, and annealing at 30 K. The spectra in panel c were measured at 4 K.

experiments with HCOOD, which is explained by a fast decay of cis-HCOOH at elevated temperatures.7 In contrast, cis-HCOOD remains quite stable up to 30 K.21 The frequencies of the bands assigned to cis-FA · · · N2 (complex III) agree well with the computational values. The νOH, νCdO, COH-COdef, and τCOH bands of the complex are shifted from the cis-FA monomer frequencies, respectively, by -20.3, -5.2, +13.2, and +53.9 cm-1 in experiment and by -22.1, -3.7, +24.1, and +70.2 cm-1 in theory. For deuterated FA, the νOD band in the cis-HCOOD · · · N2 complex III is redshifted from the monomer by -12.6 cm-1 (computationally by -16.1 cm-1), and the νC-O mode demonstrates a small blue shift both in experiment and calculation (+8.0 and +7.7 cm-1, respectively). Structure IV of the cis-FA · · · N2 complex was not identified in the experiments, which agrees with a significantly weaker interaction in this structure. The C-O stretching band

shifted from the monomer by +4.4 cm-1 (for deuterated FA) may originate from complex IV or other structures, which are not found in calculations; however, this is not supported by additional observations. Tunneling Reaction. It is seen in Figure 4a that the decay of complex III is much slower compared with the cis-FA monomer. The decay of the cis-FA monomer occurs via tunneling of hydrogen through the torsional barrier, as discussed elsewhere.7-9 Decay of the cis-FA · · · N2 complex III can also be explained by tunneling of hydrogen. Several factors can affect the tunneling rates in low-temperature rare-gas matrices.7,9,21 The cis-to-trans barrier height is obviously one of the factors,9,22 and we discuss it first in detail. It is believed that tunneling occurs at a short time scale, and no reorganization of heavy particles occurs during the tunneling time (adiabatic approximation).23 Under this concept, hydrogen tunneling in the cis-FA · · · N2 complex would break the hydrogen bond in the complex. This suggests that the configuration after the tunneling should be a trans-FA + N2 pair in a non-optimized configuration. The hydrogen bonding is practically destroyed in the transition state, as well. It follows that the (cis-FA · · · N2)to-(trans-FA + N2 pair) process has a larger barrier than the cis-to-trans process for the FA monomer, roughly by the cis-FA · · · N2 interaction energy which is -4.79 kJ/mol. Thus, we are coming to the conclusion that the decay of cis-FA · · · N2 (structure III) should be slower compared with the cis-FA monomer, in agreement with the experimental observations. An increase in the barriers in a hydrogen-bonded system has been noted by Akai et al.11 In their study, two conformers of tetrachlorohydroquinone were computationally found to be more

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stable than those of hydroquinone due to intramolecular hydrogen bonding, resulting in a difference of the torsional barrier by ∼8.4 kJ/mol at the B3LYP/6-31++G(2d,2p) level of theory. Figure 2 presents the calculated torsional barriers for the cisto-trans conversion of FA monomer, complex III, and the cis-FA-water complex (cis-FAW). The differences in barrier heights are essentially defined by the interaction energies of the cis-FA species. After BSSE corrections, the barriers for the cis-FA · · · N2 and cis-FA-water complexes are 3715 and 6540 cm-1, respectively. The differences between these values and barrier of the monomer (3233 cm-1) are comparable with the corresponding interaction energies -4.79 and -32.0 kJ/mol (400 and 2675 cm-1). It should be mentioned that the cis-to-trans torsional barrier of the FA monomer obtained in ref 15 (2676 cm-1) was calculated with relaxation of the geometry at each scan step, which mainly explains the numerical difference from the present results. We believe that our approach using the frozen coordinates is better applicable to the process of quantum tunneling. In any case, the qualitative picture concerning FA monomers remains the same. On the other hand, the previous approach is impossible to apply to the nitrogen complex III bounded through the tunneling H atom because it would imply extensive movement of the nitrogen molecule. It is interesting to compare the complexes of cis-FA with nitrogen and water. In a previous work, we have experimentally shown that the cis-FA · · · H2O complex is absolutely stable on the scale of days,13 which was explained by the efficient suppression of hydrogen tunneling. The complexation with water decreases the energy of cis-FA by -32.0 kJ/mol (our computational results), whereas trans-FA is lower in energy than cis-FA by -16.7 kJ/mol.5 In this case, hydrogen tunneling from cis-FA · · · H2O to the trans-FA + H2O pair is forbidden in the adiabatic approximation because the energy of the final system (with the same positions of the non-tunneling atoms) would be substantially higher than that of the initial system, as demonstrated in Figure 2. On the other hand, the interaction of cis-FA · · · N2 is relatively weak (interaction energy -4.79 kJ/ mol), which makes the final system energetically acceptable and the tunneling process possible. The effect of the surrounding matrix should also be commented on. As a result of tunneling, the hydrogen atom would be surrounded by argon atoms. According to Panek et al., the interaction energy between trans-HCOOH and an argon atom is -1.8 to -2.6 kJ/mol at the MP2/aug-cc-pVTZ level of theory, depending on the argon atom position.24 These interactions do not much change the situation with the cis-FA · · · H2O complex; however, this can influence the conformational change in the weakly bound FA · · · N2 system. As mentioned above, solvation effects can change the energetics of the tunneling system. In the cis-FA · · · N2 complex (structure III), the hydrogen atom of the OH group interacts with the nitrogen molecule, which weakens its interaction with the matrix compared with the uncomplexed cis-FA monomer and the transition state of the complex. Indeed, the solvation effect predicted by the PCM model for cis-FA and the transition states is more pronounced than that for cis-FA · · · N2 (structure III). The calculated dipole moments in the transition states for the monomer and complex are similar, and the solvation effects in the transition states are practically the same. The dipole moment in the transition state is smaller than in the cis-FA monomer, which relatively decreases the interaction with the matrix.7

Marushkevich et al. The PCM calculations indicate a difference of the cis-to-trans torsional barrier in an argon matrix and in vacuum. The barrier of FA monomer increases by ∼2%, whereas the opposite change is observed for the FA-nitrogen complex III, for which the barrier decreases by ∼5%. As a result, the difference in stabilization barriers of the monomer and complex changes from 780 cm-1 obtained in vacuum to 519 cm-1 in solid argon (without BSSE corrections). Thus, solvation in an argon matrix does not change correlation between the barrier height and tunneling decay time. Due to the limited accuracy of the solvation model in solids, the presented estimates should be considered qualitative. In addition to the stabilization barriers, a number of other factors can influence the tunneling efficiency. Pettersson et al. suggested that the cis-FA ground-state energy should be compared with the energy level manifold of the trans form to evaluate the energy gaps and the nature of the accepting modes involved into the tunneling process.7 Since dipole moments of the two FA conformers are substantially different (3.79 D for cis, 1.42 D for trans), the solvation energies for them are different, as well. The difference in solvation energies for the cis and trans monomers is 241 cm-1 (2.89 kJ/mol) in an argon matrix as yielded by our PCM calculations. Thus, cis-FA solvated in solid argon is higher in energy than the trans form by ca. 1120 cm-1. In this case, the smallest energy gap for the relaxation from the ground state energy of monomeric cis-FA (∼20 cm-1) is associated with the excited state of the COH-CO deformation mode (ν6) of the trans form (1103 cm-1). The energy mismatch between the initial cis and final trans levels is smaller than the Debye frequency of solid argon (65 cm-1); hence, a one-phonon process is sufficient to dissipate the excess energy, making tunneling efficient. The interaction in the cis-FA · · · N2 complex in an argon matrix lowers the energy of the ground state so that the most energetically favorable relaxation channel for cis-FA · · · N2 is connected with the first excited state of δOCO (ν7, 629.3 cm-1) and τCOH (ν9, 635.4 cm-1) modes of trans-HCOOH. The ground state of cis-FA · · · N2 is probably more than 100 cm-1 above these vibrational levels. In this case, a multiphonon process is required, in contrast to FA monomers, which slows down the tunneling process. Finally, the effect of complexation of FA with nitrogen is comparable to solvation in highly polarized media. The tunneling decay rates of cis-HCOOH in a xenon matrix are 40 and 75 min for the molecules in two matrix sites.7,25 This is close to the decay rate of the cis-FA · · · N2 complex in an argon matrix (48 min). The change in the stabilization barrier between argon and xenon matrices is ∼1 kJ/mol, which is probably insufficient to explain the strong stabilization effect.7 In solid xenon, solvation substantially decreases the energy difference between the cis and trans forms to a value below 1000 cm-1, which slows down the tunneling similarly to the case of the cis-FA · · · N2 complex. The present results agree with the semiempirical values reported in ref 21. Conclusions The geometries, interaction energies and IR spectra of complexes of FA and nitrogen have been evaluated by ab initio calculations at the MP2/6-311++G(2d,2p) level of theory. The results for the trans-FA · · · N2 complexes agree with the previous work by Lundell et al.17 Two complexes of cis-FA and nitrogen have been calculated for the first time. The computational data are presented in Tables 1-4. In experiment, the FA · · · N2 complexes are obtained by annealing of FA/N2/Ar matrices. The spectroscopic data on the

Interaction of Formic Acid with Nitrogen ground-state trans-FA · · · N2 complex agree with the previous data by Lundell et al.17 To prepare the cis-FA · · · N2 complex, we combine annealing of an argon matrix with vibrational excitation of the trans-FA monomer. This new complex has been characterized spectroscopically, and the experimental complexation-induced shifts are in a good agreement with the computational results. The experimental data are presented in Tables 1-4. The cis-FA · · · N2 complex decays in the dark by quantum tunneling of a hydrogen atom. It was found that the lifetime of cis-FA · · · N2 complex is ∼6.5 times longer than that of the cis-FA monomer at 9 K. The torsional barriers and their change upon solvation and complexation have been computationally estimated by the PCM model, confirming a higher stabilization barrier for cis-FA · · · N2 compared with the cis-FA monomer. A number of the other solvation effects, such as changes of the accepting modes and the order of the energy dissipation process, are considered and found to be in agreement with the experimental trends. The present results on the cis-FA · · · N2 complex show that intermolecular interaction can lead to the stabilization of intrinsically unstable conformers. The significant stabilization effect was previously found for trans-cis FA dimers,15,16 complexes of cis-FA with water,13 and with atomic oxygen.14 Acknowledgment. The work was supported by the Finnish Center of Excellence in Computational Molecular Science and by the Research Foundation of the University of Helsinki. The CSC IT Center for Science Ltd. is thanked for computational resources. References and Notes (1) Park, S. T.; Kim, S. K.; Kim, M. S. Nature 2002, 415, 306. (2) Lummis, S. C. R.; Beene, D. L.; Lee, L. W.; Lester, H. A.; Broadhurst, R. W.; Dougherty, D. A. Nature 2005, 438, 248. (3) Foote, J.; Milstein, C. Proc. Natl. Acad. Sci. U.S.A. 1994, 91, 10370. (4) Miyazaki, T. Atom Tunneling Phenomena in Physics, Chemistry and Biology; Springer-Verlag: Berlin, 2004. (5) Hocking, W. H. Z. Naturforsch. A 1976, 31A, 1113. (6) Pettersson, M.; Lundell, J.; Khriachtchev, L.; Ra¨sa¨nen, M. J. Am. Chem. Soc. 1997, 119, 11715.

J. Phys. Chem. A, Vol. 114, No. 39, 2010 10589 (7) Pettersson, M.; Mac¸oˆas, E. M. S.; Khriachtchev, L.; Lundell, J.; Fausto, R.; Ra¨sa¨nen, M. J. Chem. Phys. 2002, 117, 9095. (8) Mac¸oˆas, E. M. S.; Khriachtchev, L.; Pettersson, M.; Fausto, R.; Ra¨sa¨nen, M. Phys. Chem. Chem. Phys. 2005, 7, 743. (9) Khriachtchev, L. J. Mol. Struct. 2008, 880, 14. (10) Akai, N.; Kudoh, S.; Takayanagi, M.; Nakata, M. Chem. Phys. Lett. 2002, 356, 133. (11) Akai, N.; Kudoh, S.; Nakata, M. J. Phys. Chem. A 2003, 107, 3655. (12) Nishino, S.; Nakata, M. J. Phys. Chem. A 2007, 111, 7041. (13) Marushkevich, K.; Khriachtchev, L.; Ra¨sa¨nen, M. J. Phys. Chem. A 2007, 111, 2040. (14) Khriachtchev, L.; Domanskaya, A.; Marushkevich, K.; Ra¨sa¨nen, M.; Grigorenko, B.; Ermilov, A.; Andrijchenko, N.; Nemukhin, A. J. Phys. Chem. A 2009, 113, 8143. (15) Marushkevich, K.; Khriachtchev, L.; Lundell, J.; Ra¨sa¨nen, M. J. Am. Chem. Soc. 2006, 128, 12060. (16) Marushkevich, K.; Khriachtchev, L.; Lundell, J.; Domanskaya, A.; Ra¨sa¨nen, M. J. Phys. Chem. A 2010, 114, 3495. (17) Lundell, J.; Ra¨sa¨nen, M.; Latajka, Z. Chem. Phys. 1994, 189, 245. (18) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, ReVision E.01; Gaussian Inc.: Wallingford, CT, 2004. (19) Boys, S. F.; Bernardi, F. Mol. Phys. 1970, 19, 553. (20) Gantenberg, M.; Halupka, M.; Sander, W. Chem.sEur. J. 2000, 6, 1865. (21) Domanskaya, A.; Marushkevich, K.; Khriachtchev, L.; Ra¨sa¨nen, M. J. Chem. Phys. 2009, 130, 154509. (22) Benderskii, V. A.; Makarov, D. E. Wight, C. A. Chemical Dynamics at Low Temperatures; Wiley: New York, 1994. (23) Atkins, P. W.; Friedman, R. S. Molecular Quantum Mechanics, 3rd ed.; Oxford University Press: Oxford, U.K., 1997. (24) Panek, J. J.; Wawrzyniak, P. K.; Latajka, Z.; Lundell, J. Chem. Phys. Lett. 2006, 417, 100. (25) Mac¸oˆas, E. M. S.; Khriachtchev, L.; Pettersson, M.; Lundell, J.; Fausto, R.; Ra¨sa¨nen, M. Vib. Spectrosc. 2004, 34, 73.

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