Article pubs.acs.org/JPCA
Tunable Diode Lasers as a Tool for Conformational Control: The Case of Matrix-Isolated Oxamic Acid Anna Halasa,† Leszek Lapinski,† Hanna Rostkowska,† Igor Reva,‡ and Maciej J. Nowak*,† †
Institute of Physics, Polish Academy of Sciences, Al. Lotnikow 32/46, 02-668 Warsaw, Poland Department of Chemistry, University of Coimbra, 3004−535 Coimbra, Portugal
‡
S Supporting Information *
ABSTRACT: A tunable diode laser was applied as a source of narrowband near-infrared light used to manipulate the structure of the molecule of oxamic acid. Monomers of the most stable conformer I of the molecule, with the trans orientation of the OCOH group and the trans orientation of the OCCO fragment, were trapped from the gas phase in low-temperature argon, neon, and nitrogen matrixes. Monomers of oxamic acid, isolated in argon or neon matrixes, were then irradiated with narrowband near-IR light from the diode laser tuned at 6833 (Ar) or 6840 cm−1 (Ne). Upon such irradiation another conformer, II, of oxamic acid was generated, with cis orientation of the OCOH group and trans orientation of the OCCO fragment. Both forms were identified by comparison of their experimental mid-IR spectra with the spectra theoretically calculated for I and II. Subsequent irradiation of the matrix at 6940 (Ar) or 6991 cm−1 (Ne), where absorption appeared in the near-IR spectrum of the photoproduct, led to photoconversion of conformer II into form I. In a series of subsequent irradiations at 6833(Ar)/6840(Ne) cm−1 and at 6940(Ar)/6991(Ne) cm−1, the population of oxamic acid molecules was selectively shifted several times from I to II and vice versa. As far as we know, this is the first reported study where a tunable diode laser source of narrowband near-IR light was used to manipulate the structure of a molecule. Spontaneous II → I transformation was observed for Ne and Ar matrixes kept in the dark and at cryogenic temperature.
1. INTRODUCTION Matrix isolation is one of the methods used in investigations of conformers, conformational equilibria and conformational interconversions.1,2 In this method, monomeric molecules are trapped from the gas phase, together with a large excess of inert gas on a substrate cooled to a cryogenic temperature. If the barriers separating potential-energy minima corresponding to distinct conformers are high enough, then the conformers populated in the gas phase are frozen in a low-temperature matrix without changing their relative populations. In the case of lower barriers, a conversion to lower-energy conformational structures occurs during freezing the molecules into a lowtemperature environment.3,4 The presence of different conformers in a matrix can be best proven when a manipulation of relative populations of these species is feasible. Prior to matrix deposition, the equilibrium ratio of conformers in the gas phase can be shifted by changing the temperature of the vapor. This approach is limited to compounds having conformers differing in energies by no more than 8 kJ mol−1, so that they have non-negligible populations in the gas phase at moderate temperatures.4−6 Several methods can be employed to manipulate relative populations of conformers already frozen in a low-temperature matrix. For instance, UV irradiation can induce alteration of the population ratio of conformers trapped in a matrix.7−9 It was also observed for several species (such as glycolic acid10,11) that excitation with broadband NIR/IR radiation (from the IR source of the © XXXX American Chemical Society
spectrometer or from an external source) can lead to conformational changes. Upon absorption of NIR/IR light, molecules promoted to the first or second excited vibrational state can change their conformational structure. Although, with this method, the possibility of manipulation of the populations of conformers is limited, exposure of the matrixes to broadband NIR/IR radiation allowed generation of conformers, which, due to their very low population in the gas phase at thermal equilibrium, have not been previously observed. Application of tunable narrowband NIR light sources brings the investigations of conformational landscapes of matrixisolated molecules onto a qualitatively higher level. Using narrowband NIR (or in some cases mid-IR) tunable light sources it is possible to excite, in a selective way, only molecules adopting a particular conformational structure. Such an approach, pioneered by Pettersson, Lundell, Khriachtchev, and Räsänen,12 allowed effective investigation of conformers of several compounds as, e.g., formic acid,12,13 acetic acid,14 oxalic acid,15 and hydroxyacetone.16 For each of these compounds, high-energy conformers (which have never been previously observed) were photoproduced, stabilized in low-temperature Special Issue: Markku Räsänen Festschrift Received: February 10, 2014 Revised: March 7, 2014
A
dx.doi.org/10.1021/jp501448m | J. Phys. Chem. A XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry A
Article
Table 1. Relative Electronic Energies (ΔEel, kJ mol−1), and Relative Electronic Energies with ZPE Correction (ΔEtotal = ΔEel + ΔZPE) of Oxamic Acid Conformers, Calculated Using the QCISD/6-31++G(d,p) Method at Geometries Optimized at the Same Levela
matrixes and spectrally characterized. Unfortunately, investigations of this kind are still scarce. The reason for that is the high cost of tunable narrowband NIR light sources. In the studies carried out so far, optical parametric oscillators (OPO) pumped by frequency-doubled or tripled light emitted by solidstate Nd:YAG lasers were applied. Because of the high cost of such experimental setups, investigations of conformational transformations, carried out by combined usage of matrixisolation technique and narrowband tunable NIR/IR laser light sources, are performed only in a few laboratories in the world. In the current work, we investigated the conformational transformations in oxamic acid monomers isolated in lowtemperature argon and neon matrixes. Within this study, the structural changes in oxamic acid molecules were induced by excitation with monochromatic near-infrared light provided by a tunable diode laser. As far as we know, this is the first report on a successful application of a tunable diode laser as a source of NIR light inducing conformational changes in matrix-isolated molecules.
ΔEel ΔEtotal
I
II
III
0.0 0.0
12.8 13.1
20.7 20.3
a
Absolute calculated energies of form I are equal to −357.5647553 hartree (Eel) and −357.502663 hartree (Etotal).
Yang, Parr correlation functional.20 At the optimized geometries, the harmonic (or anharmonic) vibrational frequencies and IR intensities were calculated at the same DFT(B3LYP) level. The theoretical normal modes of the two lowest-energy forms of oxamic acid were analyzed by carrying out potential energy distribution (PED) calculations, performed according to the procedure described in refs 22 and 23. The set of internal coordinates used in the PED analysis was defined following the recommendations of Pulay et al.24 These coordinates are listed in Table S1, in the Supporting Information. Cartesian force constants were transformed into the force constants with respect to the molecule-fixed internal coordinates. Potentialenergy-distribution matrices have been calculated, and the elements of these matrices greater than 10% are given in Tables S2 and Table S3 (Supporting Information). All quantum-mechanical computations were carried out with the standard 6-31++G(d,p) and 6-311++G(d,p) basis sets using the Gaussian 09, Revision A.02, program.25
2. EXPERIMENTAL SECTION The sample of oxamic acid (purity 98%) used in the present study was a commercial product supplied by TCI Europe. To obtain the low-temperature matrixes, the crystalline powder of the compound was heated to ca. 370 K in a miniature glass oven placed in the vacuum chamber of a helium-cooled cryostat. The vapors coming out of the oven were codeposited with a large excess of inert gas (argon, neon, or nitrogen) onto a CsI window, cooled to 5.5 or 15 K by a closed-cycle Sumitomo SRDK-408D2 refrigerator. The mid-IR spectra were recorded with a resolution of 0.5 cm−1 using a Thermo Nicolet Nexus 670 FTIR spectrometer equipped with a KBr beam splitter and a DTGS detector. Near-IR spectra were recorded with a resolution of 1 cm−1 using the same spectrometer but equipped with a CaF2 beam splitter and an InGaAs detector. The conformational transformations were induced in monomers of oxamic acid by irradiation with continuouswave, narrowband, near-IR light provided by a tunable diode laser Toptica DL100 pro design (fwhm
