Conformational Isomerizations by Rotation around CC or CN Bonds: A

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A: Spectroscopy, Molecular Structure, and Quantum Chemistry

Conformational Isomerizations by Rotation around CC or C-N Bonds: A Comparative Study on Matrix-Isolated Glycolamide and N-Hydroxyurea Excited with Near-IR Laser Light Leszek Lapinski, Igor Reva, Hanna Rostkowska, António Jorge Lopes Jesus, Sandra Monica Vieira Pinto, Rui Fausto, and Maciej J. Nowak J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.9b01992 • Publication Date (Web): 03 Apr 2019 Downloaded from http://pubs.acs.org on April 4, 2019

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The Journal of Physical Chemistry

Conformational Isomerizations by Rotation around C-C or C-N Bonds: a Comparative Study on Matrix-Isolated Glycolamide and N-Hydroxyurea Excited with Near-IR Laser Light

Leszek Lapinski,† Igor Reva,*‡ Hanna Rostkowska,† A. J. Lopes Jesus,§ Sandra M. V. Pinto,∂ ‡ Rui Fausto‡ and Maciej J. Nowak*† †Institute

of Physics, Polish Academy of Sciences, Al. Lotnikow 32/46, 02-668 Warsaw, Poland

‡CQC,

Department of Chemistry, University of Coimbra, 3004-535 Coimbra, Portugal

§ CQC,

Faculty of Pharmacy, University of Coimbra, 3004-295, Coimbra, Portugal

∂Scuola

Normale Superiore, Piazza dei Cavalieri, 7, I-56124 Pisa, Italy

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ACS Paragon Plus Environment

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Abstract Conformers and near-IR-induced conformational transformations were studied for monomers of glycolamide isolated in low-temperature matrixes. Two conformational isomers of the compound, Tt and Cc, were trapped from the gas phase into solid Ar matrixes. Selective near-IR excitation of glycolamide molecules adopting the Tt form led to the Tt → Cc conformational conversion. Analogously, selective near-IR excitation of Cc conformers resulted in the Cc → Tt transformation. Monochromatic near-IR light, generated by frequency-tunable laser sources, was used for irradiation of matrix-isolated monomers. NearIR-induced Tt → Cc and Cc → Tt conformational transformations occurred upon excitation of 2OH, 2aNH2 and 2sNH2 overtones, as well as upon excitation of aNH2 + sNH2 combination modes. In spite of the structural similarity of glycolamide and N-hydroxyurea, no conformational conversions were observed for monomers of the latter compound isolated in Ar matrixes and excited with near-IR light. The comparison of the effects of near-IR excitations of glycolamide and N-hydroxyurea demonstrates that transformations involving rotation of molecular fragments around a single C-C bond occur much easier than transformations involving rotation of the fragments around a C-N bond. The efficiency of the latter conversions is extremely low.

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1. INTRODUCTION

Matrix isolation combined with selective excitation of isolated species with monochromatic near-IR laser light is an efficient method used in investigations of conformational equilibria and photoinduced conformational transformations.1-4 Using this technique, near-IR-induced conformational conversions were observed for a number of carboxylic acids (e.g., formic, acetic, oxalic, furoic and thiazole acids)5-10 as well as for small aminoacids (e.g., glycine and alanine).11-13 Significantly less attention has been devoted to investigations of conformers of organic amides.14-16 A picture emerging from the reports published hitherto can suggest that it is enough to excite a molecule with near-IR light and thereby provide it with energy (ca. 7000 cm−1 or 84 kJ mol−1) sufficient for overcoming a barrier separating the conformers (lower than 50 kJ mol−1), to induce a conformational conversion involving mutual rotation of molecular fragments. But is it always so? Does near-IR excitation of flexible molecules always lead to changes in conformational structure? In the current work we approach this issue by performing a comparative experimental study on the conformational behavior of two structurally similar compounds, glycolamide and N-hydroxyurea, isolated in low-temperature matrixes and excited with monochromatic near-IR light. Glycolamide can be treated as an isomer of aminoacid glycine with interchanged positions of the amino and hydroxyl groups. In a previous millimeter-wave study of glycolamide molecules seeded in a supersonic jet,15 two conformers of the compound (Chart 1) have been identified. These two forms differ in mutual torsion of the amide and hydroxymethyl fragments around the central C-C bond and also by torsion of the OH group around the single C-O bond. Analogous conformers were also observed17 for molecules of N-hydroxyurea, which have an NH group, instead of a CH2 group of glycolamide, see Chart 1. According to our current research, for matrix-isolated molecules of glycolamide, the near-IR induced conformational conversions occurred easily, but for N-hydroxyurea they did not. In both cases, the energy provided to the molecules with vibrational excitation was much higher than the torsional barrier separating the conformers. The comparison of the observed behavior of near-IR-excited glycolamide and N-hydroxyurea allowed drawing conclusions and fill the knowledge gap about the relative efficiency of conformational transformations involving torsion of heavy molecular fragments around the C-C and C-N bonds.

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2. EXPERIMENTAL SECTION Glycolamide (98%) and N-hydroxyurea (98%) used in the current study were the commercial products supplied by Sigma-Aldrich. Solid sample of a studied compound was placed in a miniature glass oven located inside the vacuum chamber of a cryostat equipped with a closedcycle refrigerator APD Cryogenics DE-202A. In the matrix-isolation experiments, vapor of a studied compound, coming out from the oven heated by resistive wire to 340 K (glycolamide)18 or to 440 K (N-hydroxyurea), was deposited onto a cold (15 K) CsI substrate, together with large excess of argon (purity N60, supplied by Air Liquide). Mid-infrared (midIR) spectra of the matrixes were recorded in the 4000-400 cm−1 range, with 0.5 cm−1 resolution, using a Thermo Nicolet 6700 FTIR spectrometer equipped with a KBr beam splitter and an MCT-B or DTGS detector. The spectra in the near-infrared (near-IR) range were collected, with 1 cm−1 resolution, using the same spectrometer, but equipped with a CaF2 beam splitter and an MCT-B or InGaAs detector. Monomers of glycolamide or N-hydroxyurea isolated in Ar matrixes were irradiated with monochromatic, frequencytunable near-IR light generated either as the idler beam of a Quanta-Ray MOPO-SL optical parametric oscillator (FWHM 0.2 cm−1, pulse energy 10 mJ), pumped with a pulsed Nd:YAG laser (10 ns, repetition rate 10 Hz), or in the continuous-wave tunable diode laser Toptica DL100 pro design (FWHM < 1MHz, 40 mW). The wavelengths of the near-IR light were measured with HighFinesse WS-5 wavelength meter. In a separate experiment, matrixisolated N-hydroxyurea was also irradiated with UV light from an HBO 200 high-pressure mercury lamp. This lamp was fitted with a water filter and a cutoff filter transmitting light with  > 270 nm. 3. COMPUTATIONAL DETAILS All quantum-chemical calculations reported in this work were carried out using the Gaussian 09 program package (revision D.01).19 In the first step of the computations, the geometries of the conformational isomers of glycolamide and N-hydroxyurea were fully optimized at the DFT level of approximation, with the B3LYP functional20-22 and the standard 6-311++G(d,p) basis set. Subsequently, the structures corresponding to the located potential-energy minima were fully optimized, using either the DFT(B3LYP) or MP2 method23 combined with the 6-311++G(3df,3pd) basis set, or the QCISD method24 combined with the 6-311++G(2d,2p) 4


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basis set. For each of the considered conformers, harmonic vibrational spectra were computed at the same DFT(B3LYP) or MP2 level of theory at which geometry was optimized. The nature of stationary points was confirmed by analysis of the corresponding Hessian matrices. These computations provided also the relative Gibbs free energies of the conformers, which were used to estimate the conformational composition of the gaseous compounds prior to the matrix deposition. Mid-IR spectra of glycolamide or N-hydroxyurea conformers trapped in Ar matrixes were interpreted by comparison of the experimental absorption bands with the harmonic wavenumbers and IR intensities computed at the DFT(B3LYP)/6-311++G(3df,3pd) level. In order to account for the basis set limitations, vibrational anharmonicity, the neglected part of electron correlation and matrix shifts, the computed harmonic wavenumbers were scaled by 0.95 (for values higher than 3000 cm−1) and by 0.98 (for values lower than 3000 cm−1). Near-IR spectra of glycolamide and N-hydroxyurea conformers considered in the current work were theoretically simulated in the DFT(B3LYP)/6-31++G(d,p) anharmonic vibrational computations, carried out at geometries optimized at the same level. A fully automated second-order vibrational perturbative approach of Barone and co-workers25,26 was applied in these calculations. This approach allows evaluation of anharmonic vibrational wavenumbers and anharmonic infrared intensities up to two quanta, including overtones and combination transitions.26-28 The computed anharmonic vibrational wavenumbers were not scaled. 4. RESULTS 4.1. Conformers of Glycolamide and their Relative Energies. The conformational structure of glycolamide is defined by the internal rotations around the C−C and O−C single bonds. Electronic energy of glycolamide was calculated, at the DFT(B3LYP)/6311++G(3df,3pd) level, as a function of the O−C−C=O and H−O−C−C dihedral angles. In the performed calculations, both O−C−C=O and H−O−C−C dihedral angles were incremented, with steps of 20°, over the full 0–360° range. At each of the 324 points, the values of O−C−C=O and H−O−C−C dihedrals were frozen while all the remaining geometrical parameters were optimized. For every partially optimized geometry electronic energy was evaluated. The resulting two-dimensional potential energy surface (PES) is presented in Figure 1. Two energy minima are identified on this surface: one of them with both dihedrals 5



equal to 0°, and the other with both dihedrals equal to 180°. The two minima are hereafter abbreviated as Cc [C, c = cis (0°)] and Tt [T, t = trans (180°)], respectively. The capital letters define the orientation around the O−C−C=O dihedral angle, and the lower case letters refer to the orientation around the H−O−C−C dihedral. A brief glance on the PES shown in Figure 1 reveals that the Cc minimum is located at a bottom of a deep and narrow potential energy well, whereas the Tt minimum lies within a shallow potential energy valley. The relative energy along this broad valley, extending from ca. 60° to 300° along the H–O–C–C coordinate, varies only within 3 kJ mol−1. This peculiarity of the potential energy surface around Tt has several implications, which are discussed below. The electronic energies (Eel) of the Cc and Tt conformers (both having Cs symmetry) were calculated for geometries fully optimized at the DFT(B3LYP), MP2 and QCISD levels. The results of these computations are presented in Table 1. All the applied methods predict Cc as the lowest-energy conformer of glycolamide. The computed electronic energy of the Tt conformer is higher by 2.17 (QCISD), 2.48 (MP2) or 3.29 kJ mol−1 (B3LYP). The value obtained in the current DFT(B3LYP)/6-311++G(3df,3pd) calculation is close to that (3.13 kJ mol–1) previously computed at the DFT(B3LYP)/aug-cc-pVTZ level.15 Inclusion of the zeropoint vibrational energy correction has a little effect on the MP2 and DFT(B3LYP) relative energies of Cc and Tt (Table 1). Intramolecular O−H···O=C (Cc) or N−H···O−H (Tt) hydrogen-bond-like interactions are present in the structures of both conformers (see Chart 1 and Figure S1 in the Supporting Information). The geometry parameters, characterizing the relative positions of the fragments involved in these energy-lowering interactions, can be extracted from the QCISD optimized geometries. In the Cc structure, the H···O distance in the O−H···O=C fragment (2.02 Å) is shorter than the H···O distance in the N−H···O−H fragment (2.16 Å) of the Tt form. Also the (O−H···O) angle in Cc (120.2°) is closer to linearity than the (N−H···O) angle in Tt (104.8°). This shows that the stabilizing hydrogenbond-like interaction in Cc is stronger than in Tt, which helps to explain the higher stability of the Cc conformer with respect to the Tt form.29 As shown in Table 1, the difference of Gibbs free energy (G) between the two conformers at 340 K (1.7 kJ mol−1) is significantly smaller than the difference of electronic energies (with or without corrections for zero-point-vibrational energy). This should be attributed to a large entropic effect that arises in Tt due to a large-amplitude, low-frequency torsional vibration along the broad potential-energy valley visible in Figure 1. According to G value, computed at the highest of the applied levels (Table 1), the equilibrium Boltzmann 6


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populations of the two conformers in the gas phase at 340 K were estimated to be 55% (Cc) and 45% (Tt). The fact that the higher-energy conformer Tt is predicted to be considerably populated in gas-phase does not necessarily mean that the Tt form has to be populated in low-temperature matrixes. The possibility of the capture of this higher-energy conformer into a cryogenic matrix is also determined by the magnitude of the energy barrier for the Tt  Cc conversion. The computed height of this energy barrier is near 14 kJ mol–1 (see symbol X in Figure 1). This value is high enough to prevent transformation of Tt into Cc (conformational cooling)30 during temperature lowering in the process of deposition of glycolamide monomers from the gas phase into a low-temperature matrix. 4.2. Infrared Spectra of Glycolamide Monomers Isolated in Ar matrixes. The mid-IR spectra of glycolamide monomers isolated in an Ar matrix are presented in Figure 2 and in Figure S2 (in the Supporting Information). In the 3700 - 3400 cm−1 region of the midIR spectrum, the highest-wavenumber band was found at 3669 cm−1. Comparison of the experimental spectrum with the spectra simulated for Cc and Tt conformers of glycolamide (Figure 2 and Table S1 in the Supporting Information) indicates that this band should be assigned to the OH stretching vibration (OH) of the Tt form. A OH band at a similarly high wavenumber, 3670 cm−1, was observed in the IR spectrum of the AAT conformer of glycolic acid isolated in an Ar matrix,2 see Chart S1 (in the Supporting Information). Such high spectral positions (3669 and 3670 cm−1) are characteristic of OH groups not involved (as H-atom donors) in a hydrogen-bond-like interaction with lone-electron pair of any heteroatom in the molecule.31,32 The appearance of the medium-strong band at 3669 cm−1 in the spectrum of matrixisolated glycolamide is a clear spectral indication of a significant population of the Tt form trapped in solid argon. It is also clear that the Tt conformer cannot be responsible for all the bands observed in the 3700 - 3400 cm−1 spectral range. The number of these bands is higher than three IR absorptions predicted to appear in this region of the spectrum of a single Tt conformer. As it is illustrated in Figure 2, the experimental mid-IR spectrum of matrixisolated glycolamide is well-reproduced by a superposition of the spectra theoretically predicted for Tt and Cc forms of the compound. This shows that Tt as well as Cc forms of glycolamide are trapped, in similar amounts, from the gas phase into low-temperature argon matrixes. One of the bands attributed to the spectrum of the Cc conformer appears at 3481 cm−1. The frequency of this band, assigned to the OH vibration, is close to that of the analogous OH band (at 3505 cm−1) observed in the spectrum of the structurally similar Cc 7



conformer of hydroxyacetone,33 see Chart S1 (in the Supporting Information). In both glycolamide and hydroxyacetone, the Cc conformers have the OH group involved in a stabilizing interaction with the carbonyl group. This interaction, closing a five-membered intramolecular ring (Chart 1), results in lowering the frequency of the OH stretching vibration.29 The IR bands due to the antisymmetric aNH2 vibrations of the amino group appear in the spectra of both Cc and Tt conformers at similar wavenumbers 3549 / 3547 cm−1, so that these experimental bands partially overlap. Analogously, the bands due to symmetric sNH2 vibrations in Cc and Tt also have very close spectral positions 3431 and 3427 cm−1 and do partially overlap. The experimental IR bands observed in the lower-wavenumber (1800 - 500 cm−1) region of the spectrum are fairly well reproduced by the spectra theoretically predicted for Cc and Tt conformers. The broad band appearing in the spectrum at ca. 1200 cm−1 (Figure 2) may be assigned to the bending ( OH) vibration in the Tt form (Table S1 in the Supporting Information). The broad shape of this band can be explained by a shallow potential extending over a wide range of dihedral angles describing the torsional movement of the OH group in Tt (Figure 1). The bands due to 2OH, 2aNH2 and 2sNH2 overtones were observed in the near-IR region of the infrared spectrum of glycolamide isolated in an Ar matrix (Figure 3). In addition to that, the bands due to the (a+s)NH2 combination mode were also detected in this spectral range. The observed near-IR absorption bands are in fair agreement with theoretical predictions based on the performed anharmonic calculations. For the Tt conformer, the bands owing to 2OH, 2aNH2, (a+s)NH2, and 2sNH2 vibrations were found in the experimental spectrum at 7169, 7041, 6845 and 6767 cm−1, whereas for Cc the analogous bands appeared at 6743, 7036, 6854 and 6780 cm−1, respectively. The spectral effects of selective irradiations of the matrix with narrowband near-IR light allowed unambiguous separation of the near-IR absorptions due to Tt and Cc forms (Figure 3 and section 4.3). 4.3. Conformational Conversions Induced in Matrix-Isolated Glycolamide Monomers by Excitation with Narrowband, Near-IR Laser Light. Monomers of glycolamide isolated in Ar matrixes were subjected to a series of irradiations with monochromatic, tunable near-IR light. For every irradiation, the laser was tuned to a wavenumber corresponding to an absorption band observed in the near-IR spectrum of the matrix-isolated compound (Figure 3). Irradiations at 7169, 6845 and 6767 cm−1 resulted in conversion of the Tt conformer into the Cc form (Figures 3 and 4). Irradiations at 7036, 6854, 8


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6780 and 6743 cm−1 led to the conformational transformation in the opposite Cc → Tt direction (Figures 3, 4 and Figure S3 in the Supporting Information). The experimental results show that selective near-IR excitations of the 2OH, 2aNH2 and 2sNH2 overtones, as well as the (a+ s)NH2 combination mode, induce the Tt → Cc or Cc → Tt transformation involving mutual torsion of the amide and hydroxymethyl fragments of the molecule around the C−C bond. For the structurally similar molecule of glycolic acid, mutual torsion of the carboxylic and hydroxymethyl fragments was reported to be the most probable scheme of the near-IRinduced SSC → GAC conformational conversion.2 However, it is difficult to provide experimental evidence that would exclude an alternative, hypothetical mechanism involving direct hydrogen-atom transfer from one oxygen atom to the other, within the carboxylic group of glycolic acid, see Scheme S1 in the Supporting Information. In the currently considered case of the near-IR-induced Tt → Cc or Cc → Tt conversions in glycolamide, there is no room for speculations about alternative schemes involving hydrogen-atom jumps. The conformational Cc ↔ Tt conversion in glycolamide can occur only by the torsion of the hydroxymethyl group with respect to the amide fragment. To complete the Tt → Cc or Cc → Tt conformational transformations, the torsion around the central C-C bond needs to be accompanied by the torsion of the OH group within the hydroxymethyl fragment (Scheme 1). 4.4. Monomers of N-Hydroxyurea Isolated in Argon Matrixes; Effects of Near-IR Excitation with Narrowband Laser Light. According to the theoretical calculations,17,34-36 the most stable form of N-hydroxyurea is the conformer with two oxygen atoms placed in trans (T) orientation with respect to the C-N bond connecting the C=O and N(H)OH fragments (see Chart 1 and Figure S4 in the Supporting Information). The computations, carried out in the current work at the DFT(B3LYP), MP2 and QCISD levels (Table 2), predict that the energy of this conformer should be lower by 9.1–9.6 kJ mol–1 than the energy of the form with the two oxygen atoms in the cis (C) conformation (Chart 1). The difference between the computed Gibbs free energies of the two conformers at 440 K (temperature of the gaseous compound before the matrix deposition) amounts to 7.8 – 8.7 kJ mol–1, which corresponds to relative Boltzmann populations equal to ca. 91% T(E) and 9% C(Z). Symbols E and Z refer to the nomenclature used in several previous works.17,34-37 The mid-IR spectrum, recorded in the current work for N-hydroxyurea isolated in an Ar matrix (Figure 5 and Figure S5 in the Supporting Information), is virtually the same as that reported by Sałdyka.17 Following the interpretation presented in the previous work,17 the 9



relatively strong bands (e.g. those at 3590/3593, 3557/3560 and 3441/3443 cm−1) should be assigned to the dominating T(E) conformer, whereas the weak bands (e.g. those at 3535 and 3431 cm−1) should be assigned to the less populated C(Z) conformer. The bands due to the more populated T(E) form of N-hydroxyurea are split into two components, that most probably reflects trapping of this form in two spectrally distinct matrix sites. A fragment of the near-IR spectrum, recorded in the present work for N-hydroxyurea isolated in solid Ar, is shown in Figure 6. Matrix-isolated monomers of N-hydroxyurea were irradiated with narrowband near-IR light, generated in a laser tuned to wavenumbers corresponding to the absorption features presented in Figure 6. Irradiations at 7009, 7002, 6998, 6866 and 6794 cm−1 led to intensity changes within the doublet profiles of the bands assigned to the T(E) form (Figure 5 and Figures S5 and S6 in the Supporting Information). This can be attributed to rearrangement of matrix cavities, induced by energy provided to these microenvironments with near-IR excitation of the trapped molecules. Upon irradiation of matrix-isolated N-hydroxyurea at 7009, 7002, 6998, 6866 and 6794 cm−1, intensity changes of the bands assigned to the minor C(Z) conformer were extremely small or just negligible (Figure 5). This demonstrates that the efficiency of the near-IR-induced T(E) → C(Z) process is extremely low. Similarly to the effects of near-IR irradiations of matrix-isolated N-hydroxyurea, exposure of monomers of the compound trapped in solid Ar to UV ( > 270 nm) light resulted only in intensity changes within the doublet profiles of the bands due to the T(E) form (Figure S5 in the Supporting Information). Intensities of the bands attributed to the C(Z) conformer did not change upon UV ( > 270 nm) irradiation of the matrixes. 5. CONCLUDING DISCUSSION The experiments carried out in the current work showed that selective near-IR excitations of the 2OH, 2aNH2 and 2sNH2 overtones or the (a+s)NH2 combination vibrations of the Cc conformer of glycolamide lead to Cc → Tt conformational transformation. It has also been experimentally demonstrated that selective near-IR excitation of analogous overtones or combination vibrations of the Tt conformer of the compound induces reverse Tt → Cc transformation. Conformational conversions in both directions involve mutual torsion of the hydroxymethyl (CH2-OH) and the amide (O=C−NH2) fragments around the single C−C bond connecting these groups (Scheme 1). This is the first report of such a conformational 10


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conversion occurring in an amide molecule. Hitherto, near-IR-induced conformational conversions occurring by torsion of molecular fragments around the C−C bond have been reported for several compounds: hydroxyacetone,3 glycine,11 glycolaldehyde,38 glycolic acid,2 kojic acid,39 and propionic acid.40 These experimental results, together with those obtained in the current work for glycolamide, provide convincing evidence demonstrating that near-IRexcitation of 2OH or 2NH2 overtones of OH or NH2 groups attached in an  position (may lead to conformational transformation involving torsion of fragments around the central single C-C bond. However, it is not always so. For example, near-IR excitations of 2OH overtones of GAC and AAT conformers of glycolic acid2 did not result in transformation of these forms into the most stable SSC conformer.2 As far as we know, the only reported case of the near-IR-induced conformational conversion occurring by torsion of molecular fragments (bigger than two hydrogen atoms in NH2) around the C-N bond concerns the transformation observed for thiosemicarbazide.41 Although for this compound the near-IR-induced conformational transformation involving torsion around the C-N bond did occur, it was of very low efficiency. Upon prolonged (4.5 h) irradiation with strong (500 mW) laser radiation, only a very tiny fraction of matrix-isolated molecules was transformed from one conformational structure to the other. The IR spectral changes, reflecting this conformational conversion, were as small as 10−3 of absorbance units, which corresponds to changes of IR band intensities by 1%. In the present work, attempts to observe a near-IR-induced conformational transformation were also made for N6-methyladenine isolated in Ar matrixes. The two conformers of this compound differ from each other in torsion of the methylamino group around the C-N bond connecting the H-N-CH3 fragment and the purine ring. Near-IR excitations of 2N9H and 2N6H overtones did not result in transformation of one rotamer of N6-methyladenine into the other. Similarly, no (or extremely inefficient) near-IR-induced conformational conversion was observed for N-hydroxyurea studied in the current work. All this suggests that there is a large and systematic difference between the efficiencies of nearIR-induced conformational transformations occurring by torsion of molecular fragments around the C-C and C-N bonds. In this respect, comparison of the structurally similar molecules of glycolamide and N-hydroxyurea can be especially instructive. The obvious parameter, which should be taken into consideration in attempts to explain the experimentally observed difference between the behavior of near-IR-excited glycolamide and N-hydroxyurea, is the height of the barriers for torsion around the C-C and 11



C-N bonds. The results of theoretical estimation of these barriers are presented in Figure 7 and Figure S7 (in the Supporting Information). The computed barrier for torsion around the C-C bond in glycolamide is of moderate height 17.4 kJ mol−1. Similar or lower values (5 - 18.6 kJ mol−1) were predicted for other compounds such as glycine, hydroxyacetone, glycolic and kojic acids, where the near-IR-induced conformational transformations occur by torsions of hydroxymethyl, aminomethyl or carboxylic fragments around single C-C bonds.2,3,11,39 The barrier for torsion around the C-N bond in N-hydroxyurea is substantially higher (45.3 kJ mol−1). The latter value is close to that (45 kJ mol−1) predicted for torsion around the C-N bond in thiosemicarbazide.41 Hence, for near-IR-induced conformational transformations involving large displacements of "heavy" atoms (such as N or O), it looks that barriers as high as 45 kJ mol−1 constitute obstacles, which significantly hinder (or make impossible) torsions of molecular fragments with respect to each other. This does not concern conformational transformations where the only fragment changing its position is the light hydrogen atom. Near-IR-induced conformational transformations occurring by hydrogen atom displacement (torsion of an OH group) were observed e.g. for oxamic acid16 and thiazole carboxylic acid,10 although these processes require crossing the barriers as high as 54 - 63 kJ mol−1. In these latter cases, hydrogen atom tunneling is expected to facilitate the change of conformational structure. Certainly, the height of the barrier is not the only parameter that governs the conformational conversions induced by near-IR excitation. The mechanisms of vibrational energy transfer from the initially excited stretching 2OH or 2NH2 modes to the torsional movement around the C-C or C-N bond should also be taken into account as one of the important factors. Since the mechanisms of vibrational energy transfer are very difficult for theoretical modeling, there is still room for a search of empirical regularities governing nearIR-induced conformational transformations.

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Table 1. Theoretical Relative Energies (kJ mol−1) and Relative Populations at 340K, Calculated for Cc and Tt conformers of Glycolamide.

Cc

Tt

DFT(B3LYP) Eel E0 G Pop. (%)

0.00 0.00 0.00 64.4

3.29 3.00 1.68 35.6

0.00 0.00 0.00 64.3

2.48 2.42 1.66 35.7

0.00 0.00 0.00 54.9

2.17 1.88 0.56 45.1

MP2 Eel E0 G Pop. (%) QCISD Eel E0a Ga Pop. (%)

Eel - relative electronic energy; E0 - relative electronic energy corrected for zero-point vibrational energy (E0 = Eel + ZPVE); G - relative Gibbs free energy at 340 K; all relative energies were calculated with respect to the energy of the most stable conformer Cc. Equilibrium populations (Pop) were estimated from the Boltzmann distribution using the G values. The 6-311++G(3df,3pd) basis set was applied in the DFT(B3LYP) and MP2 calculations; the 6-311++G(2d,2p) basis set was applied in the QCISD calculations. a

E0 and G calculated using vibrational frequencies and rotational constants computed at

the DFT(B3LYP) level.

13



Table 2. Theoretical Relative Energies (kJ mol−1) and Relative Populations at 440K, Calculated for T(E) and C(Z) conformers of N-hydroxyurea.

T(E)

C(Z)

0.00 0.00 0.00 91.5

9.53 8.56 8.70 8.5

0.00 0.00 0.00 89.5

9.09 7.83 7.85 10.5

0.00 0.00 0.00 91.7

9.64 8.67 8.81 8.3

DFT(B3LYP) Eel E0 G Pop. (%) MP2 Eel E0 G Pop. (%) QCISD Eel E0a Ga Pop. (%)

Eel - relative electronic energy; E0 - relative electronic energy corrected for zero-point vibrational energy (E0 = Eel + ZPVE); G - relative Gibbs free energy at 440 K; all relative energies were calculated with respect to the energy of the most stable conformer T(E). Equilibrium populations (Pop) were estimated from the Boltzmann distribution using the G values. The 6-311++G(3df,3pd) basis set was applied in the DFT(B3LYP) and MP2 calculations; the 6-311++G(2d,2p) basis set was applied in the QCISD calculations. a

E0 and G calculated using vibrational frequencies and rotational constants computed at

the DFT(B3LYP) level.

14


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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Tables and figures showing optimized geometries, relative energies and the barrier heights for the low-energy conformers of glycolamide and N-hydroxyurea; figure and table with the infrared spectrum of glycolamide and its vibrational assignment; figures presenting the effects induced by near-IR irradiations of glycolamide and the effects induced by near-IR and UV irradiations of N-hydroxyurea; chart showing the structures of glycolamide conformers compared with the structurally similar forms of hydroxyacetone and of glycolic acid; scheme of conformational transformations in glycolic acid and glycolamide. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected] ORCID Igor Reva: 0000-0001-5983-7743 Maciej J. Nowak: 0000-0002-0693-4109 Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work has been supported by the bilateral Project No. 2505 for cooperation between Poland and Portugal, and by the Project POCI-01-0145-FEDER-028973 funded by the Portuguese “Fundação para a Ciência e a Tecnologia” (FCT). I.R. acknowledges the FCT for the Investigador FCT grant. The Coimbra Chemistry Centre (CQC) is supported by FCT through the Project No. UI0313/QUI/2013, also co-funded by FEDER/COMPETE 2020-EU.

15



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17



(24) Pople, J. A.; Head-Gordon, M.; Raghavachari, K. Quadratic Configuration Interaction. A General Technique for Determining Electron Correlation Energies. J. Chem. Phys. 1987, 87, 5968-5975. (25) Barone, V. Anharmonic Vibrational Properties by a Fully Automated Second-Order Perturbative Approach. J. Chem. Phys. 2005, 122, No. 014108. (26) Bloino, J.; Barone, V. A Second-Order Perturbation Theory Route to Vibrational Averages and Transition Properties of Molecules: General Formulation and Application to Infrared and Vibrational Circular Dichroism Spectroscopies. J. Chem. Phys. 2012, 136, No. 124108. (27) Barone, V.; Biczysko, M.; Bloino, J. Fully Anharmonic IR and Raman Spectra of Medium-Size Molecular Systems: Accuracy and Interpretation. Phys. Chem. Chem. Phys. 2014, 16, 1759-1787. (28) Barone, V.; Bloino, J.; Guido, C. A.; Lipparini, F. A Fully Automated Implementation of VPT2 Infrared Intensities. Chem. Phys. Lett. 2010, 496, 157-161. (29) Rozenberg, M.; Shoham, G.; Reva, I.; Fausto, R. A Correlation between the Proton Stretching Vibration Red Shift and the Hydrogen Bond Length in Polycrystalline Amino Acids and Peptides. Phys. Chem. Chem. Phys. 2005, 7, 2376-2383. (30) Reva, I.; Lopes Jesus, A. J.; Rosado, M. T. S.; Fausto, R.; Eusébio, M.E.; Redinha, J.S. Stepwise Conformational Cooling towards a Single Isomeric State in the Four Internal Rotors System 1,2-Butanediol. Phys. Chem. Chem. Phys. 2006, 8, 5339-5349. (31) Han, S. W.; Kim, K. Infrared Matrix Isolation Study of Acetone and Methanol in Solid Argon. J. Phys. Chem. 1996, 100, 17124-17132. (32) Coussan, S; Bouteiller, Y.; Perchard, J. P.; Zheng, W. Q. Rotational Isomerism of Ethanol and Matrix Isolation Infrared Spectroscopy. J. Phys. Chem. A 1998, 102, 5789-5793. (33) Sharma, A.; Reva, I.; Fausto, R. Matrix Isolation Study and Ab Initio Calculations of the Structure and Spectra of Hydroxyacetone. J. Phys. Chem. A 2008, 112, 5935-5946. (34) Di Gregorio, G.; La Manna, G.; Paniagua, J. C.; Vilaseca, E. Conformational Analysis of N-Hydroxyurea in the Gas Phase. J. Mol. Struct. (Theochem) 2004, 673, 87-92. (35) Remko, M.; Lyne, P. D.; Richards, W. G. Molecular Structure, Gas-Phase Acidity and Basicity of N-Hydroxyurea. Phys. Chem. Chem. Phys. 1999, 1, 5353-5357. (36) La Manna, G.; Barone, G. The Molecular Structure of N-Hydroxyurea. Int. J. Quant. Chem. 1996, 57, 971-974. 18


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(37) Kolasa, T. The Conformational Behaviour of Hydroxamic Acid. Tetrahedron 1983, 39, 1753-1759. (38) Duvernay, F.; Butscher, T.; Chiavassa, T.; Coussan, S. IR Induced Photochemistry of Glycolaldehyde in Nitrogen Matrix. Chem. Phys. 2017, 496, 9-14. (39) Halasa, A.; Reva, I.; Lapinski, L.; Rostkowska, H.; Fausto. R.; Nowak, M. J. Conformers of Kojic Acid and Their Near-IR-Induced Conversions: Long-Range Intramolecular Energy Transfer. J. Phys. Chem. A 2016, 120, 2647-2656. (40) Maçôas, E. M. S.; Khriachtchev, L.; Pettersson, M.; Fausto, R.; Räsänen, M. Internal Rotation in Propionic Acid: Near-Infrared-Induced Isomerization in Solid Argon. J. Phys. Chem. A 2005, 109, 3617-3625. (41) Rostkowska, H.; Lapinski, L.; Kozankiewicz, B.; Nowak, M. J. Photochemical Isomerizations of Thiosemicarbazide, a Matrix Isolation Study. J. Phys. Chem. A 2012, 116, 9863-9871.

19



Chart, scheme and figures with captions

Chart 1. Molecular structures of glycolamide and N-hydroxyurea conformers.

20


Page 20 of 29

Page 21 of 29

0

E /

kJ mol

4

8

12

16

20

24

28

32

36

1

300 OCC=O dihedral angle / degree

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


240

Tt

180 120 X

60 0 -60 -60

Cc

0

60

120

180

240

300

HOCC dihedral angle / degree

Figure 1. Contour plot of potential energy calculated for glycolamide molecule as a function of the O–C–C=O and H–O–C–C dihedral angles. Both angles were incrementally fixed in steps of 20°, while all remaining parameters were fully optimized at the DFT(B3LYP)/6311++G(3df,3pd) level of theory. The minima are indicated in the plot as Cc and Tt (see the structures in Chart 1). The color bar on top represents the relative energy scale (zero at this scale is assigned to the energy computed for the global minimum, Cc). The isoenergy lines are spaced by 2 kJ mol–1. Symbol x indicates the transition state interconnecting the Cc and Tt minima.

21



~

300

Tt OH

sNH2

100

aNH2

200

OH

IR Intensity / km mol

-1

Cc

b 0 0.4

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 29

~

Ar, 15 K

0.2

a 0.0 3700

3500

1800

1600 Wavenumber / cm

1400

1200

1000

-1

Figure 2. (a) Experimental mid-IR spectrum of glycolamide monomers isolated in a lowtemperature argon matrix at 15 K; compared with (b) harmonic wavenumbers and IR intensities calculated at the DFT(B3LYP)/6-311++G(3df,3pd) level for: (red) Cc conformer, (blue) Tt conformer of glycolamide. The theoretical wavenumbers higher than 3000 cm−1 were scaled by a factor of 0.95, whereas those lower than 3000 cm−1 were scaled by 0.98. Symbols “~” indicate truncated bands.

22


2OH

Tt

0

2sNH2

-1

( a + s) NH2

2aNH2

{

2

{

4

2OH

6

{

b

6743

6780

6767

0.001

6845

0.002

6854

7036

7169

0.003

c

Cc

-2

0.004

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


IR intensity / km mol

Page 23 of 29

a 0.000 7300

7150

7000

6850

6700

-1

Wavenumber / cm

Figure 3. (a) Experimental near-IR spectrum of glycolamide monomers isolated in an argon matrix at 15 K. Red and blue arrows indicate the wavenumbers at which matrix-isolated glycolamide monomers were excited with monochromatic near-IR light; (b) difference spectrum: the spectrum recorded after irradiation of the matrix at 7036 cm−1 minus the spectrum recorded before that irradiation; (c) theoretical anharmonic near-IR spectra calculated at the DFT(B3LYP)/6-31++G(d,p) level for: (red) Cc conformer and (blue) Tt conformer of glycolamide. The theoretically calculated wavenumbers were not scaled. Theoretical intensities calculated for Cc were multiplied by −1.

23


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Absorbance


3700

7169 cm

1

6743 cm

1

7036 cm

1

6845 cm

1

6854 cm

1

6767 cm

1

6780 cm

1

3600

3500

Wavenumber / cm

Page 24 of 29

3400

1

Figure 4. Spectral indications of conformational transformations induced in monomers of glycolamide by monochromatic near-IR excitations of their 2OH, 2aNH2, 2sNH2 overtones or (a+s)NH2 combination modes. Glycolamide molecules isolated in argon matrixes were excited with near-IR laser light tuned to wavenumbers corresponding to absorption bands observed in the near-IR spectrum (Figure 3). Each of the spectra presented in the current figure was obtained as a difference: the spectrum recorded after irradiation at the specified wavenumber minus the spectrum recorded before that irradiation. Irradiations at (6743, 7036, 6854 and 6780 cm−1, red) resulted in the Cc → Tt transformation, whereas irradiations at (7169, 6845 and 6767 cm−1, blue) led to conformational conversions in the opposite Tt → Cc direction. 24


Page 25 of 29

c

0.4

0.3

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.2

b

T(E) T(E)

0.1

T(E) C(Z)

C(Z)

0.0 3600

3550

3500

a

3450

Wavenumber / cm-1

Figure 5. Fragment of the mid-IR spectrum of N-hydroxyurea trapped in an argon matrix at 15 K: (a) spectrum recorded before any irradiation; (b) difference spectrum: the spectrum recorded after monochromatic near-IR irradiation of the matrix at 7009 cm−1 minus the spectrum recorded before that irradiation; (c) difference spectrum: the spectrum recorded after monochromatic near-IR irradiation at 6866 cm−1 minus the spectrum recorded before that irradiation. Dashed rectangles indicate the spectral positions of the bands due to the C(Z) conformer, where no absorbance changes appear in the difference spectra b and c.

25



7002 cm-1 Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 29

0.008 0.006

7009 cm-1

6998 cm-1 6866 cm-1

0.004

6794 cm-1

0.002 0.000 7000

6800

Wavenumber / cm

6600 -1

Figure 6. Fragment of the near-IR spectrum of N-hydroxyurea trapped in an argon matrix at 15 K. Arrows indicate wavenumbers at which the matrix was irradiated with monochromatic near-IR light.

26


Page 27 of 29

TS (C/T) 45.3

-1

oxyur ea

40

30

N-hyd r

E / kJ mol

20

(Tt/Cc)

10

0

9.5

0.0

C(Z)

Cc

mid e

17.4

Gly col a

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Tt T(E)

3.3 0.0

Figure 7. Potential energy diagram showing: (blue) energies of Cc and Tt conformers of glycolamide and the energy of the transition state between them; (red) energies of C(Z) and T(E) conformers of N-hydroxyurea and the energy of the transition state on the minimumenergy path connecting these forms. Energies of the most stable forms of glycolamide [Cc] and N-hydroxyurea [T(E)], were assigned to zero at the relative energy scales. The energies presented in this diagram were computed at the DFT(B3LYP)/6-311++G(3df,3pd) level of theory.

27



H

O

O

N

HH

H

H

7036, 6854, 6780, 6743 cm-1

Page 28 of 29

7169, 6845, 6767 cm-1

H

Cc

O

H H

N O

H

H Tt

Scheme 1. Near-IR-induced conformational transformations observed for monomers of glycolamide isolated in low-temperature argon matrixes.

28


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TOC Graphic

29


Conformational Isomerizations by Rotation around CC or CN Bonds: A

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