Matrix-Isolation Studies of Noncovalent Interactions - ACS Publications

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Matrix-Isolation Studies of Noncovalent Interactions: More Sophisticated Approaches Leonid Khriachtchev* Department of Chemistry, University of Helsinki, P.O. Box 55, FI-00014 Helsinki, Finland ABSTRACT: Noncovalent interactions are crucial for many physical, chemical, and biological phenomena. Matrix isolation is a powerful method to study noncovalent interactions, including hydrogen-bonded species, and it has been extensively used in this field. However, there are difficult situations, such as in the case of species that are impossible to prepare in the gas phase. In this article, we describe some advanced approaches allowing studies of complexes that are problematic for the traditional methods. Photolysis of a suitable precursor in a matrix can lead to a large concentration of 1:1 complexes, which are otherwise very difficult to prepare (e.g., the H2O···O complex). Photolysis of species combined with annealing can lead to complexes of molecules with mobile atoms (e.g., the same H2O···O complex). Simultaneous photolysis of two species combined with annealing can produce complexes of radicals via reactions of the photogenerated complexes with mobile atoms (e.g., the H2O···HCO complex). Interaction of noble-gas (Ng) hydrides with other species is another topic (e.g., the N2···HArF complex) and very large blue shifts of the H−Ng stretching modes are normally observed for these systems. Complexes and dimers of the higher-energy conformer of formic acid have been prepared by using selective vibrational excitation of the ground-state conformer. The higher-energy conformer of formic acid can be efficiently stabilized in the complexes with strong hydrogen bonding. We also consider some problematic cases when the changes in the vibrational frequencies of the 1:1 complexes are very small (e.g., the phenol···Xe complex) and when the complex formation is prevented by strong solvation in the matrix (e.g., species in solid xenon).

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INTRODUCTION Noncovalent interactions are crucial for understanding the structure and properties of matter and constitute the basis of supramolecular chemistry.1−10 Many physical, chemical, and biological phenomena such as the formation of molecular crystals and biopolymers, solvation dynamics, protein folding, catalysis, and molecular recognition are attributed to noncovalent interactions with typical stabilization energies from ∼4 to 80 kJ mol−1. These interactions are responsible for the existence of liquid phase; thus, they are crucial for the science of life. Intermolecular complexes, particularly with water, are important for atmospheric chemistry because they can change the radiative balance and reaction channels in the atmosphere.11−13 Understanding the molecular mechanisms of these complicated processes requires studies of relatively simple model systems, and this research has been very active both experimentally and theoretically. The structure of molecules and the charge distribution are changed by noncovalent interactions. As a result, the frequencies of the vibrational transitions of the interacting molecules differ from those of the monomers. This effect makes vibrational spectroscopy a powerful method to investigate intermolecular complexes. These spectral shifts can be also simulated theoretically, allowing one to analyze the geometries of the complexes.14−23 Numerous studies of intermolecular complexes have been performed in supersonic jets (see, for example, refs 24−33) and other media (see, for example, refs 34−43). A very interesting finding is the existence of blueshifting hydrogen bonds.17,37,42 © 2015 American Chemical Society

In this article, we concentrate on an experimental technique of matrix-isolation infrared (IR) spectroscopy. This method of research was invented to study isolated species, especially those with low energetic stability and high chemical reactivity.44−46 An inert matrix and low temperature allow one to measure vibrational spectra of isolated species over an extended period of time without essential perturbation from the environment; thus, this method provides detailed and reliable information on the vibrational properties of the species. The matrix-isolation method is particularly very suitable to study noncovalent interactions, including hydrogen-bonded species. This effect is commonly analyzed by comparing the vibrational spectra of the monomers and complexes. A number of methods can be used to prepare intermolecular complexes in cryogenic matrixes. Most traditionally, the intermolecular complexes are prepared by adding two species to the matrix gas and depositing the matrix at somewhat elevated temperatures and/or annealing the matrix after deposition to mobilize the species. As modifications of this method, more than one gaseous mixture can be simultaneously deposited onto a cold substrate and, moreover, interacting molecules can be deposited from solid samples located in the vacuum chamber. Very many systems have been studied in this way (see, for example, refs 47−67). In most cases, the experimental IR Received: December 2, 2014 Revised: January 27, 2015 Published: February 13, 2015 2735

DOI: 10.1021/jp512005h J. Phys. Chem. A 2015, 119, 2735−2746

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The Journal of Physical Chemistry A

water, and the calculated H···O hydrogen bond length is 2.23 Å.73 This complex is presumably formed in the in-cage reaction of two OH radicals similarly to the known gas-phase reaction.74 The identified H2O···O complex is a good example of an interacting pair, which is very difficult to isolate in matrixes by using the traditional method whereas it is the main channel of UV photolysis of H2O2 in an Ar matrix. This complex was also identified upon photolysis of hydrogen peroxide in Kr and Xe matrixes.75 In these matrixes, the yield of its formation is smaller than in an Ar matrix because the O atom can with some probability escape from the parent cage upon UV photolysis. A surprising observation was made for the matrix-isolated H2O···O complex.71,73,75 The 193 nm photolysis produces a larger amount of this complex (more than 50% of the initial amount of H2O2 molecules in an Ar matrix) compared to photolysis at longer wavelengths. If this photolyzed matrix is irradiated at longer wavelengths (for example, at 300 nm), the amount of the H2O···O complex decreases and hydrogen peroxide partially recovers (Figure 1), which introduces a new H2O···O + hν → H2O2 photoreaction. The efficiency of this reaction in an Ar matrix has the maximum at 275 nm.71 In the original article, it has been suggested that the first step of this reaction is a charge transfer in the H2O···O complex; i.e., the water molecule becomes positively charged and the oxygen atom carries a negative charge.71 Then, these ions react and the formed intermediate (probably oxywater H2O−O) converts to hydrogen peroxide via the hydrogen shift. Simple energetic estimates confirm this model. More detailed analysis of this reaction, which is consistent with the simple consideration, can be found elsewhere.76 The 193 nm photolysis of formaldoxime (CH2NOH) in an Ar matrix produces a large number of new bands, which can be separated into four groups.77 The assignment is greatly helped by selective excitation of the vibrations above 3100 cm−1 by narrowband IR light from an optical parametric oscillator. It appears that the produced species are two pairs of complexes, and the members of the same pair can be interconverted to each other. One pair of the complexes is formed between HCN and water whereas the second pair is between HNC and water. The stronger complexes are formed by interaction of the water oxygen with the hydrogen of HNC (the strongest one) and HCN. The complexes formed by interaction of a water hydrogen atom with the nitrogen and carbon atoms are weaker in terms of interaction energy. Once again, the type of hydrogen bonding can be changed by excitation of the stretching modes of the complex units, which is a useful experimental method as also demonstrated elsewhere.78−80 The HCN···water complexes are constituted by stable molecules; however, it is hardly possible to obtain such high concentration of these complexes by codeposition of HCN and water. HNC is a minor tautomer of HCN and its deposition from the gas phase is obviously problematic. The 193 nm photolysis of propiolic acid (HCCCOOH) in various matrixes (Ar, Kr, and Xe) leads to several products.81 The first product is the higher-energy (cis) conformer of propiolic acid, which decays to the more stable (trans) form presumably via tunneling of the hydrogen atom through the torsional barrier. The other photoinduced bands are divided into three groups. The main product (up to 50% as estimated in a Kr matrix) is the H2C2···CO2 complex with the parallel geometry, which is computationally lower in energy than the linear structure by 2.9 kJ mol−1. As in the example above, this complex is constituted by stable molecules; however, it is hardly

spectroscopy (and sometimes Raman spectroscopy) has been accompanied by quantum chemical calculations. However, this traditional strategy is less suitable for the molecules that are difficult (or impossible) to prepare in the gas phase, for example, for highly reactive and unstable species. In addition, this method leads to relatively small amounts of the 1:1 complexes with an interference of monomers and larger clusters. In the present article, we consider approaches to prepare “exotic” complexes, which are highly problematic for the traditional method. In most cases, the preparation method includes photochemical processes and annealing-induced mobilization of photolysis fragments. We also consider other difficult cases when, for example, the complex formation is prevented by strong solvation in a matrix and when the complex has no spectroscopic fingerprints. It should be mentioned that unusual complexes can be also prepared by deposition of matrixes through a microwave discharge;68 however, this interesting method is not considered in this article. Deposition of species through discharge is more suitable for the formation of unusual charged species,69 whereas the formation of neutral complexes is less controllable compared to the approaches described below. Unusual complexes can be prepared using laser ablation;70 however, the description of this method is also beyond the scope of this article.

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PHOTOLYSIS AND PHOTOCHEMISTRY Photolysis of hydrogen peroxide (H2O2) in an Ar matrix at wavelengths shorter than ∼300 nm leads to two products,71 one of which is a pair of isolated OH radicals absorbing at 3554 cm−1 (Figure 1). OH radicals are the dominating products of

Figure 1. FTIR spectra in the OH stretching region of hydrogen peroxide and the photolysis products in an Ar matrix: (a) spectrum after deposition; (b) difference spectrum with respect to spectrum a showing the result of 193 nm photolysis; (c) difference spectrum with respect to spectrum in (b) showing the result of irradiation at 300 nm. Adapted with permission from ref 73. Copyright 1998 American Chemical Society.

the photolysis of hydrogen peroxide in the gas phase;72 thus, their appearance in the matrix-isolation experiment is reasonable. In addition, these results show that an OH radical can exit the cage with a notable probability, which is relatively rare for molecular photoproducts in solid matrixes. The second product has two absorptions in the water stretching region at 3731 and 3633 cm−1 that are shifted from the ν3 and ν1 bands of nonrotating water by about −5 cm−1 (Figure 1). These bands have been assigned to the H2O···O complex where the ground-state oxygen atom interacts with a hydrogen atom of 2736


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the HCO monomer bands after annealing of the photolyzed matrix.83 The quantum chemical calculations predict three structures of the HCO···H2O complex (Figure 3). The analysis

possible to obtain such high concentration of this complex by codeposition of H2C2 and CO2. Moreover, in a Xe matrix, this complex does not appear after codeposition of H2C2 and CO2 at all and we will come back to this question later in this article. Two other photolysis channels lead to the complexes between ethynol (HCCOH) and CO and between water and C3O. The relative amounts of these two products do not exceed 5%.

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PHOTOLYSIS COMBINED WITH ANNEALING Thermal annealing can mobilize small species in a solid matrix. Notable long-range mobility of O atoms in a Kr matrix occurs at temperatures above 20 K, and the activation energy of this process has been measured to be ∼70 meV76 and between 50 and 130 meV. 82 In a representative experiment, a H2O/N2O/Kr matrix was photolyzed at 193 nm (Figure 2).76 Figure 3. Calculated structures of the HCO···H2O complex. Structure III is the main product identified in the experiment. Adapted with permission from ref 83. Copyright 2013 American Chemical Society.

of the IR spectroscopic data shows that the main experimental product has the cyclic structure III with two hydrogen bonds. In the case of the HCO···DOH complex, the hydrogen bond is formed by the D atom, which is consistent with the general predictions for noncovalent interactions of deuterated species.88

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COMPLEXES OF NOBLE-GAS HYDRIDES Noble-gas hydrides are artificial molecules having the formula HNgY where H = hydrogen atom, Ng = noble-gas atom, and Y = electronegative fragment.89 The examples of these species include HArF, HKrCl, HXeBr, etc. The HNgY molecules are formed in the reaction of the neutral fragments H + Ng + Y. The general method of their preparation includes photolysis of the HY precursor in an Ng matrix and subsequent thermal mobilization of the H atoms that can react with the Ng···Y centers. The strong H−Ng stretching absorption (computationally >1000 km mol−1) makes their experimental detection relatively easy. Theoretical analysis shows that these species are characterized by the strong ionic character (HNg)+Y−. The H−Ng bond is mainly covalent whereas the Ng−Y bong is mainly ionic. The HNgY molecules are metastable with respect to the Ng + HY global energy minimum, but they are protected from the decomposition by relatively high barriers. All experimentally observed HNgY molecules are lower in energy that the H + Ng + Y asymptote, which allows their diffusioncontrolled formation at low temperatures.90 The HNgY molecules have been prepared in cryogenic matrixes;89 thus, the traditional preparation of their complexes by codeposition from the gas phase is evidently problematic. In addition, noncovalent interactions of these molecules are supposed to be very interesting due to their weak bonding and large dipole moments, which ensures a large complexation effect. Another related topic is solvation of these species in polarizable media, which may lead to their stabilization;91 however, this important issue is beyond the scope of this article. To prepare an HNgY···M complex, one can first stabilize the HY···M complex in a solid matrix by codepositing the HY and M molecules with excess of the matrix gas (panel I in Figure 4 where the case of the (HY)2 dimer is shown).92 The next step (panel II) is photolysis of the HY···M complex, which produces H atoms and the Y···M complexes in the Ng matrix (Y···HY in Figure 4). Thermal annealing of the matrix mobilizes the H

Figure 2. Photolysis and annealing of a H2O/N2O/Kr (1/1/700) matrix: (a) FTIR spectrum after deposition; (b) difference spectrum with respect to spectrum a showing the result of 193 nm photolysis; (c) difference spectrum with respect to spectrum b showing the result of annealing. Adapted with permission from ref 76. Copyright 2007 American Chemical Society.

This radiation mainly decomposes N2O, producing O atoms and inert N2 molecules whereas the H2O concentration does not change much. Upon annealing at ∼25 K, the O atoms move in the matrix and are attached to water forming the H2O···O complex. As judged by the IR spectrum, it is exactly the same species, which is formed upon UV photolysis of hydrogen peroxide in a Kr matrix,75 but it is now prepared in a different way. Similarly to the earlier works, the H2O···O complex is efficiently converted to hydrogen peroxide by irradiation at 300 nm. The yield of the synthesis of hydrogen peroxide exceeds 50% with respect to the number of decomposed N2O molecules. This method of the complex preparation can be further developed. A more advanced photochemical method comprises simultaneous photolysis of two species followed by thermal mobilization of one of the photolysis products that reacts with the other photolysis products. A HCOOH/HY/Kr matrix (Y = Cl and Br) was photolyzed at 193 nm.83 The main photolysis product of HCOOH in a Kr matrix is the CO···H2O complex,84 and photodecomposition of HY produces H atoms.85 Annealing of the photolyzed matrix at temperatures above ∼27 K mobilizes H atoms in a Kr matrix,86,87 and they can react with the CO···H2O complex producing the HCO···H2O complex. Indeed, new bands appear in the IR spectra near 2737


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kJ mol−1). In this complex, the positive charge of the (HKr)+ unit increases by about +0.10e compared to the monomer. The HCl frequency in the HKrCl···HCl complex is strongly redshifted upon complexation (down to ∼2373 cm−1, shift of about −500 cm−1). For the HKrCl···(HCl)2 complex, the corresponding shifts are tentatively found to be +724 and −718 cm−1 (Figure 5). These huge effects presumably originate from the very weak bonding of HKrCl. This hypothesis was systematically confirmed for the HXeY···H2O complexes in a Xe matrix (Y = Cl, Br, and I).95 The experimentally observed complexes were assigned to structure A with the OH···Y hydrogen bond (Figure 6). It

Figure 4. General method of the preparation of the HNgY complexes in a matrix: (I) deposition of the HY···M complex (in the present figure, the HY dimer); (II) photolysis of the matrix; (III) annealing of the matrix; (IV) formation of the HNgY···M complex (in the present figure, the HNgY···HY complex). The HNgY···M complex can have different structures (see, for example, Figure 6).

atoms (panel III), and they can react with the neutral Ng···Y···M triads forming the HNgY···M complex (panel IV). The examples of complexes prepared in this way include HArF···N2, HKrCl···HCl, HXeBr···H2O, etc.93−95 An important observation on the HNgY complexes is the blue shift of the H−Ng stretching mode.92 The strongest effect has been observed for the HKrCl···HCl complex with the H−Kr stretching frequency of up to 1782 cm−1 compared to the HKrCl monomer absorption at 1476 cm−1 (Figure 5),

Figure 6. Calculated structures of the HXeY···H2O complexes (Y = Cl, Br, and I). The strongest complex has structure A, and it is the main experimental product. Reproduced with permission from ref 95. Copyright 2014 American Institute of Physics.

appears that the complexation effect is stronger on the less stable molecule (HXeI) and weaker on the more stable molecule (HXeCl). In fact, the complexation-induced shifts of the H−Xe stretching mode are +82 cm−1 for Y = Cl (6.7% of the frequency of the monomer), +101 cm−1 for Y = Br (9.1%), and +138 cm−1 for Y = I (14.1%). This trend is not due to the strength of the interaction that shows the opposite order: the strongest interaction for Y = Cl and the weakest interaction for Y = I. The CCSD(T) interaction energies for structure A are −23.9, −21.5, and −18.5 kJ mol−1 for Y = Cl, Br, and I, respectively. As mentioned above, the blue shift of the H−Ng stretching mode presumably demonstrates the stabilization of the H−Ng bond. For example, the complexes of HXeOH with one and two water molecules have been identified experimentally in a Xe matrix with the H−Xe stretching shifts of +103 and +164 cm−1 from the monomeric frequency (1578 cm−1) in a good agreement with the MP2 calculations.96 However, this stabilization of the H−Ng bond does not necessarily mean the stabilization of the entire molecule. It was theoretically suggested for the HXeOH···(H2O)n complexes that the stabilization barrier along the bending coordinate decreases for larger n, and it practically disappears for n = 3.96 This theoretical result raises doubts about possible preparation of HXeOH in water ice, at least at ambient pressure. In the previous examples, the precursor of the HNgY···M complex has been the HY···M complex. Two exceptions from

Figure 5. FTIR difference spectra demonstrating the result of annealing at 30 K for two HCl/Kr matrixes with the 1/100 and 1/1000 concentration ratios. The matrixes were deposited at 35 K and photolyzed at 193 nm. The spectra were measured at 8.5 K. The HKrCl···HCl bands are marked with C, and the HKrCl···(HCl)2 bands are marked with T. Adapted with permission from ref 94. Copyright 2009 American Chemical Society.

which means a giant blue shift of +306 cm−1 (>20%).94 It follows that the H−Kr bond force constant increases almost by half upon the complex formation! The strengthening of the H− Ng bonds and the blue shifts of the H−Ng stretching mode has been explained by the enhancement of the ionic character (HNg)+Y− in the complexes.92 It should be noted that the H− Ng bond may not be participating in the interaction to produce this effect like in the most stable HKrCl···HCl complex with the Cl···HCl hydrogen bond (interaction energy of about −40 2738


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with the low-temperature limit, which is a fingerprint of tunneling reactions.106 H atom tunneling has been found in a number of other matrix-isolation studies.107−111 Two approaches can be used to prepare intermolecular complexes containing cis-FA. First, vibrational excitation of trans-FA monomer in a matrix produces cis-FA and annealing of the matrix mobilizes the species, which leads to the cis-FA···M complex:

this procedure have been found. The first case is the HXeCCH···CO2 complex.97 In fact, the needed precursor H2C2···CO2 was not formed in a Xe matrix by codeposition of H2C2 and CO2. This fact can be explained by a weak interaction in this complex compared to the interaction of the monomers with matrix Xe atoms. In this situation, isolation of the H2C2 and CO2 monomers is energetically more favorable than the complex formation. We will return to this concept of strong solvation later in this article. In the present case, this problem was solved by UV photolysis of a proper species. The needed precursor H2C2···CO2 was created in significant amounts by 193 nm photolysis of propiolic acid in a Xe matrix as described above.81 This finding allows using a normal strategy to prepare the HXeCCH···CO2 complex that shows the H−Xe stretching shift of up to +32 cm−1. The second exceptional case is the HXeCCH···H 2 C2 complex.98 Indeed, the (H2C2)2 precursor is seen in a Xe matrix after deposition but no bands suitable to the HXeCCH···H2C2 complex are identified after photolysis and annealing of the matrix. It is possible that photolysis of (H2C2)2 does not lead to the required complex C2H···H2C2 due to some complicated in-cage photochemistry producing different products. Another strategy was chosen instead. It appears that H2C2 molecules become mobile in a Xe matrix upon annealing above ∼50 K. Thus, a matrix with H2C2 monomers is first prepared, a part of H2C2 molecules is photolyzed, and the matrix is annealed at 40 K, which produces the HXeCCH monomers. Annealing at temperatures above 50 K mobilizes the remaining H2C2 molecules that attack the HXeCCH monomers, which creates the HXeCCH···H2C2 complexes. The H−Xe stretching mode of this complex is shifted by +18 to +28 cm−1, in good agreement with MP2 calculations. For the HXeCCH···(H2C2)2 complex, the shift is tentatively +51 cm−1.

trans‐FA + M + hν → cis‐FA + M

(1)

cis‐FA + M + T → cis‐FA ··· M

Second, one can prepare the trans-FA···M complex in a conventional way of codeposition and its vibrational excitation can change the FA conformation, producing the cis-FA···M complex: trans‐FA + M + T → trans‐FA ··· M

(2)

trans‐FA ··· M + hν → cis‐FA ··· M

where T means annealing in approach 1 and deposition at a higher temperature or annealing in approach 2. Both approaches have been used in our experiments. By using approach 1, we have prepared complexes of cis-FA with H2O, N2, and CO2.112−114 The calculated structures of the cis-FA···CO2 complex are shown in Figure 7. In the experiments

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COMPLEXES AND DIMERS OF CIS-FORMIC ACID Matrix isolation has been extensively used to study higherenergy molecular conformations.99,100 In our group, we have mainly concentrated on conformers that are intrinsically unstable even at very low temperatures. A representative example is formic acid (HCOOH, FA), which is interesting due to the biological and environmental relevance. This molecule has two conformers, the ground-state trans form and the cis form, which is 1365 cm−1 higher in energy.101 The amount of cis-FA in the gas phase is negligible. After deposition of a matrix, practically all FA molecules accept the trans conformation. It was found in our laboratory that cis-FA in significant amounts could be prepared in a matrix by vibrational excitation of the ground-state form, and its IR spectrum was reported for the first time in an Ar matrix.102 Various vibrational transitions (fundamentals, combinations, and overtones) can be used to achieve the conformational change.103 The process is observed even for excitation of the OH and CH stretching modes, whose frequencies are somewhat smaller than the calculated trans-to-cis isomerization barrier (∼4000 cm−1).104 It has been observed that the cis-FA molecules in matrixes are not stable and decay to the ground-state trans form in the dark even at temperatures below 20 K.105 In fact, the lifetime of the cis-FA molecules in an Ar matrix at 8 K is about 7 min. The observed cis-FA decay is explained by H atom tunneling through the torsional cis−trans barrier (computationally ∼2700 cm−1 in vacuum). This mechanism is confirmed by the strong H/D isotope effect and by the specific temperature dependence

Figure 7. Calculated structures of the cis-FA···CO2 complexes. The interaction energies (kJ mol−1) are shown in parentheses, and the distances are given in Å. Structures I and II in the original article are the trans-FA···CO2 complexes (not shown). Reproduced with permission from ref 114. Copyright 2012 American Chemical Society.

in an Ar matrix, the most stable structure III (H-bonded) is found to appear after vibrational excitation combined with annealing.114 The identified cis-FA···N2 complex also has the Hbonded structure.113 The lifetimes of the cis-FA···CO2 and cis-FA···N2 complexes in an Ar matrix are 13.3 h and 48 min, respectively, which are much longer than that of the cis-FA monomer (7 min). This order of the lifetimes is consistent with the calculated tunneling barriers. These calculations are performed by scanning the torsional coordinate for the fixed positions of other atoms (adiabatic approximation). It is shown that for the H-bonded structures, the stabilization barrier increases roughly by a value of the binding energy (8.5 and 4.8 kJ mol−1 in the cis-FA···CO2 and cis-FA···N2 complexes). It 2739


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indication of the cis−cis dimers are observed after annealing of Ar matrixes containing cis-FA molecules. This fact is presumably explained by a relatively small concentration of the cis form achievable at the annealing temperatures (about 30 K) when the lifetime of cis-HCOOH considerably decreases.105 However, the situation becomes more promising for the deuterated species (HCOOD) because the tunneling rate strongly decreases (by a factor of ∼3 × 103 at 7 K) and more than 90% of FA can be converted to the cis form in an Ar matrix. Moreover, in contrast to HCOOH, the decay rate of cis-HCOOD is not affected much by annealing at 30 K.115 After the matrixes containing a large amount of cis-HCOOD were annealed, new bands appeared that were assigned to several cis−cis dimers.122 Combination of photolysis and annealing discussed above can be also used to make cis-FA complexes. A FA/N2O/Kr matrix was photolyzed at 193 nm producing O atoms from N2O, and the photolysis was stopped when an essential part of FA was not decomposed.123 The idea was to test a possible reaction between FA and O atoms. Indeed, when a matrix contains only trans-FA, thermal mobilization of O atoms leads to two reaction products, which are the trans and cis forms of peroxyformic acid (Figure 9). Surprisingly, the third product

should be mentioned that a number of other factors can also influence the tunneling rate as discussed elsewhere.105,115,116 The observed complexation-induced stabilization of the higher-energy cis-FA conformer is a remarkable effect. Is it possible to stop tunneling decay of higher-energy conformers? The answer is yes, at least at low temperatures. The cis-FA···H2O complex was found to be practically stable at low temperatures,112 which was explained by a strong hydrogen bond in this complex (30 kJ mol−1, see ref 117.). This binding energy is larger than the energy difference between the cis-FA and trans-FA monomers. It follows that the cis-FA···H2O complex is lower in energy than the trans-FA + H2O pair produced by adiabatic tunneling of the H atom through the torsional barrier; thus, the density of states at the initial energy behind the barrier is close to zero. The cis-FA···Xe complex was prepared in an Ar matrix by using approach 2, i.e., by vibrational excitation of the trans-FA···Xe complex.118 In this case, two types of complexes were identified, with the H-bonded and non-H-bonded structures. As previously, the H-bonded cis-FA···Xe complex is more stable than the cis-FA monomer (lifetimes 22 and 7 min at 4.3 K, respectively). Surprisingly, cis-FA is destabilized in the non-H-bonded complex (lifetime 2.4 min). This destabilization effect is explained by the calculated tunneling barrier of this complex, which is ∼80 cm−1 lower than that of the cis-FA monomer. A number of trans−cis dimers of FA have been identified, mainly in an Ar matrix. Two trans−cis dimers were prepared by approach 2, i.e., by vibrational excitation of the corresponding trans−trans dimers.119,120 The selective vibrational excitation leads to the rotation of the free OH group of one of the trans-FA units to the cis orientation. It is interesting that the lifetime of one of these dimers (tc1 in the original notation) in an Ar matrix is longer by a factor of about 3 than that of the cisFA monomer, and this observation is also explained by the quantum chemical calculations predicting a higher tunneling barrier in this dimer.116,119,120 Three trans−cis dimers of FA (tc2, tc3, and tc5) were prepared in an Ar matrix by approach 1, i.e., by annealing of matrixes containing both trans-FA and cis-FA monomers (Figure 8).121 In dimers tc2 and tc3, the OH

Figure 9. Reaction of formic acid with O atoms. trans-FA reacts with an O atom forming peroxyformic acid (the cis form in this figure). cisFA forms a hydrogen-bonded complex with an O atom.

appears if the matrix contains some amount of the higherenergy cis form of FA. This additional product is identified as the hydrogen-bonded cis-FA···O complex with the calculated H···O hydrogen bond length of 2.11 Å (Figure 9). This complex is found to be practically stable in the dark at matrix temperatures; however, the exposure to broadband IR light converts the cis-FA···O complex to peroxyformic acid, probably via the formation of the intermediate trans-FA···O complex. These results show that cis-FA does not react with O atoms, at least at low temperatures, in contrast to trans-FA, thus, presenting a strong case of a conformation-dependent reaction and molecular recognition. The identification of molecular complexes and structural assignment are essentially based on the comparison of measured IR spectra with quantum chemical calculations. Figure 10 presents the experimental and calculated shifts of the OH stretching frequency in different FA (both cis and trans) H-bonded complexes as a function of the calculated interaction energy. It is seen that the agreement between experiment and theory is very good with respect to the spectral shifts. In general, the H-bonded complexes with stronger interaction show larger red shifts of the OH stretching mode (see the inset for the complex with water). However, for the FA complexes with Xe, N2, and CO2, this does not occur, especially for cis-FA. For example, the cis-FA···CO2 complex is characterized by

Figure 8. Trans−cis dimers of FA prepared in an Ar matrix by a combination of vibrational excitation of trans-FA and annealing. The calculated interaction energies (kJ mol−1) are shown in parentheses. Adapted with permission from ref 121. Copyright 2011 American Chemical Society.

group of the cis-FA unit participates in strong hydrogen bonds, which increases the tunneling barrier height. In this case, the cis-FA unit is practically stable at low temperatures, similarly to the case of the cis-FA···H2O complex. Five of six theoretically found trans−cis dimers of FA have been identified in our experiments. It should be also mentioned that all six predicted trans−trans dimers of FA have been experimentally observed, four dimers being observed in our studies for the first time.120,121 The calculations also predict five cis−cis dimers of FA. However, in the experiments with natural FA (HCOOH), no 2740


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that the thermodynamically unfavorable structure can be stabilized at lower deposition temperature due to shorter solidification time of the deposited matrix material. The competition of solvation and complex formation has a spectroscopic fingerprint. The HY frequencies of the HY···N2 complexes (Y = Br and Cl) are higher in a Xe matrix than those of the Q bands of the HY monomers (by 6 and 10 cm−1 for Y = Cl and Br, respectively). This is opposite to the “normal” behavior when the HY frequency decreases upon the formation of a hydrogen bond.53,93 After deposition of a HY/N2/Xe matrix at relatively low temperatures (30−35 K), photolysis and annealing lead to the HXeY···N2 complexes (Y = Br and Cl) with shifts of the H−Xe stretching mode of ca. +10 cm−1 from the monomer values, in good agreement with the MP2 results.125 These shifts are remarkably smaller than those observed for the other HNgY complexes. For example, the shift for the HXeBr···CO2 complex in a Xe matrix is +53 cm−1, and it is up to +150 cm−1 for the HXeBr···HBr complex.126,127 The calculated interaction energies follow this trend, being −5.5, −16, and −28.5 kJ mol−1 for the HXeBr···N2, HXeBr···CO2, and HXeBr···HBr complexes, respectively. Another problematic case concerns the absence of clear spectroscopic fingerprints of intermolecular complexes that can distinguish them from the monomeric species. For example, this is the case of the complexes of phenol with xenon studied by IR absorption spectroscopy in a Ne matrix and by quantum chemical calculations.128 The structure where the Xe atom interacts with the π electron system of the aromatic ring (π complex) is theoretically the most stable 1:1 phenol···Xe structure (Figure 11), but it has no characteristic shifts from the phenol monomer in the calculated vibrational spectrum, and

Figure 10. Experimental and calculated shifts of the OH stretching mode of FA in different complexes as a function of the calculated interaction energy. The data are from refs 112−114, 117, and 118.

much stronger interaction than the cis-FA···N2 and cis-FA···Xe complexes but shows a smaller spectral shift. Theoretical analysis of different contributions to the intermolecular interaction is needed to understand this nonmonotonous dependence.

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OTHER PROBLEMATIC CASES The calculations of molecular complexes are usually performed for species in vacuum whereas the experiments under discussion are made in solid matrixes. The interaction with the matrix changes the vibrational frequencies of the complex and monomers to different extents. An interesting example is the H2O2···CO complex studied in Ar, Kr, and Xe matrixes.124 It has been found that the matrix effect on the vibration of the bonded OH group is much weaker than on that of the hydrogen peroxide monomer. The OH stretching frequency of the monomer is shifted from the gas-phase value by −20.2, −34.0, and −48.0 cm−1 in Ar, Kr, and Xe matrixes, respectively. As a result, the complexation-induced shifts differ in these matrixes (−40.1, −29.0, and −17.0 cm−1, respectively). This solvation effect may cause confusions in the structural assignment of matrix-isolated complexes, which is based on the monomer-to-complex shifts. In contrast, the shifts of the bonded OH vibration from the monomeric gas-phase value are −60.3, −63.0, and −65.0 cm−1 in these matrixes, which estimates the complexation effect more correctly. In some cases, the solid matrix can prevent the formation of weak complexes due to the strong solvation of the monomers as discussed above for the H2C2···CO2 complex in a Xe matrix. A similar problem was found when we tried to prepare the HXeY···N2 complexes (Y = Br and Cl): the needed precursor HY···N2 did not appear when a HY/N2/Xe matrix was deposited at about 45−50 K, the temperature usually used to prepare intermolecular complexes in Xe matrixes.125 Consequently, no bands suitable for the HXeY···N2 complex appear in the spectrum after photolysis and annealing. The suppression of the complex formation is probably connected with the competition of solvation and complex formation as mentioned above. However, when the matrix is deposited at 30 K, the HY···N2 band unexpectedly appears in the spectrum. It seems

Figure 11. Examples of the calculated structures of the 1:1, 1:2, and 1:3 phenol···Xe complexes and the relative intensities of the wellshifted OH stretching bands as a function of the Xe concentration in a Ne matrix. The intensities of the bands with different shifts are shown by squares (10−15 cm−1), triangles (22−28 cm−1), and circles (34−39 cm−1). The phenol/Ne concentration ratio is 1/4200. The matrixes were deposited at 4.3 K and annealed at 10 K. The spectra were measured at 4.3 K. Reproduced with permission from ref 128. Copyright 2012 American Institute of Physics. 2741


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selective excitation of various vibrational transition.77 The products of photolysis in solid matrixes can differ from the gasphase products due to the cage effect, and this allows the preparation of a variety of complexes. Photolysis of a species can lead to a small fragment (atom) that can be then mobilized by thermal annealing, which promotes its complexation or reaction with another molecule. By using this method, for example, we can prepare the H2O···O complex, the O atoms being produced by UV photolysis of N2O.76 As a development of this approach, simultaneous photolysis of two species combined with annealing leads to complexes of radicals (e.g., H2O···HCO).83 Hopefully, more examples of this approach will be found in the future. Kr and Xe matrixes are suitable for this approach because the mobility of H and O atoms can be efficiently activated in these media by annealing.76,82,86,87 It is interesting to expand these studies to other reactive atoms (for example, fluorine). We have identified a number of complexes with “exotic” species that are prepared practically exclusively in matrixisolation conditions. First, interaction of noble-gas hydrides HNgY with other molecules can be studied in solid matrixes (e.g., N2···HArF),93 and the blue shift of the H−Ng stretching mode is a normal effect in this case.92 The HY···M complex is a typical precursor for the HNgY···M complex although some exceptional cases have been found (for example, the HXeCCH···CO2 and HXeCCH···H2C2 complexes).97,98 An important question is whether it is possible to improve kinetic stability of noble-gas hydrides in a properly organized surrounding.91 The second example of an “exotic” species is the higherenergy conformer of formic acid, cis-FA, which is present in the gas phase in negligible amounts. However, many complexes and dimers of cis-FA have been prepared in low-temperature matrixes by using selective vibrational excitation of the lowerenergy (trans) form. It has been found that the higher-energy conformer can be stabilized by complexation, up to the complete stabilization in complexes with strong hydrogen bonding (for example, in the complex of cis-FA with water).112 It is interesting to expand these studies to complexes of other species fundamentally destabilized by H atom tunneling, including biomolecules. We have also discussed specific cases when the complex identification is problematic. The interaction with the matrix can change the monomer-to-complex shift, as demonstrated for the H2O2···CO complex,124 which may complicate the structural assignment. The next example is about the case of weak complexes and strong solvation in the matrix. When the HY/N2/Xe (Y = Cl and Br) matrix is deposited at temperatures above 40 K, no indication of the HY···N2 complex is observed, which is explained by the competition between the HY···N2 interaction and solvation.125 However, this weak complex appears if the Xe matrix is deposited at lower temperatures (30−35 K) and the solidification of the matrix material is relatively fast. In this case, photolysis and annealing of the matrix lead to the HXeY···N2 complexes also with very weak interaction. The simulation of the matrix effect on intermolecular complexes is a challenge for computational chemistry. Another specific case concerns complexes without characteristic spectral fingerprints, for which the direct spectroscopic evidence of the complex formation is problematic. For example, this is the case of the phenol···Xe complex with the π structure.128 In this situation, only indirect information on the formation of the 1:1 complex is obtained by analyzing the Xe

this complicates its experimental characterization. When a small amount of Xe is added to a phenol/Ne matrix, no new bands appear in the spectrum, in particular in the OH stretching region, which is due to the formation of π complex with hidden spectroscopic fingerprints. However, for larger Xe concentrations, new bands are found in the IR spectra and they were connected to the formation of the phenol···Xen (n ≥ 2) complexes. These experiments show the high efficiency of the formation of large Xe clusters in a Ne matrix that can accommodate a major part of phenol molecules. A recent study of complexes between nitrous oxide (N2O) and three representative aromatic compounds (phenol, p-cresol, and toluene) in a Ne matrix has provided direct identification and characterization of the 1:1 complexes (structure of one of these complexes appears in the picture near the abstract; reproduced with permission from ref 129, copyright 2014 American Institute of Physics).129 In these cases, the interaction is dominated by dispersion forces, and the interaction energies are relatively low (about −12 kJ mol−1); however, the complexes are clearly detected by the frequency shifts of the characteristic bands of both N2O and aromatic molecules (the methyl group). In this case, the bands of N2O are principally for identifying the complexes. This success has been recently developed for complexes between aromatic compounds (toluene and p-cresol) with Xe atoms in a Ne matrix.130 It has been shown that the methyl group of these molecules is a sensitive probe of the interaction with Xe. The bands of the 1:1 complexes of p-cresol and toluene with Xe appear at low Xe concentration and their intensities relative to the monomer bands are nearly proportional to the Xe/Ne concentration ratio. For the p-cresol···Xe system, additional OH stretching bands appear at higher Xe concentrations, which are suitable for the complexes with several Xe atoms, similarly to the phenol−Xe system.128 It should be mentioned that partially deuterated aromatic molecules are used in these studies, which provides a simpler spectrum of the characteristic methyl group. This experimental trick strongly helped the identification of the elusive complexes with the π structure. Similar complexes can be presumably formed for aromatic amino acids, for which these aromatic compounds are useful models.

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CONCLUDING REMARKS Matrix isolation is a powerful method to study noncovalent interactions, and it has been extensively used for these studies for decades.45−70 The complexation effect is commonly analyzed by comparing vibrational spectra of the complex and the monomers isolated in inert matrixes. Most traditionally, the molecular complexes are prepared by adding two species to the matrix gas and depositing the matrix at somewhat elevated temperatures and/or annealing the matrix after deposition. However, this general strategy is impossible to use when these species are not available in the gas phase. We have considered more sophisticated approaches allowing studies of complexes that are very problematic for the traditional method. For example, photolysis of a suitable precursor can lead to a large concentration of the matrixisolated complexes, which are otherwise very difficult to prepare. A good example is the H2O···O complex that is the main channel of UV photolysis of hydrogen peroxide in an Ar matrix.71,73 In the case of the H2O···HCN and H2O···HNC complexes (produced by photolysis of formaldoxime), the identification of the complex structures is greatly supported by 2742


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concentration dependences of the IR spectra. For p-cresol and toluene, the methyl group is found to be a sensitive probe of the π structures even though tricks with partial deuteration is needed to obtain reliable information on the complex formation.129,130 These results have been used for calibration of computational methods for the studies of Xe anesthesia.131 These approaches may be useful for studies of week complexes of natural amino acids and other biomolecules, which is a promising direction for matrix-isolation studies.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: leonid.khriachtchev@helsinki.fi. Tel: +358294150310. Notes

The authors declare no competing financial interest. Biography

Leonid Khriachtchev graduated from Leningrad State University, Russia, in 1981. He completed his Ph.D. study in Quantum Electronics in 1986 and became a Senior Scientist and a group leader in 1988 in the Institute of Physics at the same university. At that time, his research dealt with resonant light pressure and optical pumping of the ground state of atoms in the gas phase. In 1994, he joined the University of Helsinki, Finland, and he is currently a Senior Scientist in the Chemistry Department at this university. His scientific activities are focused on experimental spectroscopy of various molecular species and nanoscale materials. He has contributed to the construction of new chemical systems at low temperatures, including noble-gas compounds and high-energy conformers. Silicon-based photonics has been another important part of his research activity. He is the editor of the books “Silicon Nanophotonics: Basic Principles, Present Status, and Perspectives” (Pan Stanford Publishing, 2008) and “Physics and Chemistry at Low Temperatures” (Pan Stanford Publishing, 2011).

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ACKNOWLEDGMENTS The author thanks all his colleagues who contributed to the presented results and whose names appear in the original publications. This work has been partially supported by the Academy of Finland Project KUMURA (No. 1277993).

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REFERENCES

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