A study of hydrogen bonded vibrational spectra of (R)-(+)-Methylsuccinic acid, as aided by DFT dimer analysis

    A study of hydrogen bonded vibrational spectra of (R)-(+)-methylsuccinic acid, as aided by DFT dimer analysis J. Tonannavar, Yashaswita B. Chavan, Jayashree Yenagi PII: DOI: Reference:

S1386-1425(16)30066-X doi: 10.1016/j.saa.2016.02.011 SAA 14277

To appear in: Received date: Revised date: Accepted date:

7 September 2015 5 February 2016 14 February 2016

Please cite this article as: J. Tonannavar, Yashaswita B. Chavan, Jayashree Yenagi, A study of hydrogen bonded vibrational spectra of (R)-(+)-methylsuccinic acid, as aided by DFT dimer analysis, (2016), doi: 10.1016/j.saa.2016.02.011

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ACCEPTED MANUSCRIPT A study of hydrogen bonded vibrational spectra of (R)-(+)-Methylsuccinic acid, as aided by DFT dimer analysis J. Tonannavar*, Yashaswita B. Chavan and Jayashree Yenagi

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Vibrational Spectroscopy Group, Department of Physics,

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Karnatak University, Dharwad – 580 003, India

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* Corresponding author: Tel: +919448375426

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E-mail address: [email protected]

Abstract:

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Infrared and Raman spectral measurements in the region 4000 – 400 cm-1 have been carried out for (R)-(+)-Methylsuccinic acid. The vibrational band structures near 3100 – 3040 cm-1 in the IR and near 1650 cm-1 in the Raman spectra have indicated the presence of an inter-molecular

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hydrogen bonding. A DFT dimer model has been proposed that involves O-H•••O=C type of

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hydrogen bonding. The proposed dimer model has been derived from the three stable monomers computed at RHF/3-21G and B3LYP/6-311+G(d,p) levels of theory. A total of six dimer

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structures have been considered with Boltzmann population of 38% for the most stable dimer and 62% for the remaining five dimer populations. A Boltzmann population weighted vibration spectra has predicted bands, among others, for O-H•••O=C group that are in very good agreement with experiment. All the dimers have the same structure in that the two pairs of -O-H and -O=C form a closed cyclic structure with a local center of inversion. This dimer geometry has given rise to one asymmetric mode at 1683 and one symmetric –C=O mode at 1637 cm-1 corresponding to mutually exclusive an experimental IR band at 1700 and a Raman band at 1651 cm-1. Further, the bond length, H•••O, for the most stable dimer is 1.686 Å, being shorter than the sums of van der Waals radii, 2.72 Å and the angle between O-H and H•••O is almost linear (1790) suggest a that the hydrogen bonding is fairly strong.

Keywords: (R)-(+)-Methylsuccinic acid, O-H•••O=C hydrogen bonding, Density Functional Theory, IR, Raman.

ACCEPTED MANUSCRIPT Introduction: Hydrogen bonding continues to attract scientists on account of its fundamental role in determining structural, physical and chemical properties of molecules. It is a determinant in

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understanding structural behaviour of water, proteins and DNAs as derived from recent studies of strong, low-barrier hydrogen bond in enzymatic reactions [1]. It has been shown that the

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spectroscopy of molecular solvation, that is, gas-phase aromatic solute-(solvent)n clusters with

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several water or methanol molecules, as macroscopic phenomenon is best understood in terms of different types of hydrogen bonds [2]. Vibrational spectroscopy of stretching mode of hydroxyl,

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-OH, as a probe in unraveling structure and dynamics in hydrogen-bonded systems in relation to water interface and solvation is of contemporary interest [3]. Based on Bader’s Atoms in

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Molecules theory (AIM), Monteiro and Firme have demonstrated that the stability of alkanes complexes is the result of the inter- and intra-molecular hydrogen-hydrogen bonding (H-H bonding) [4]. The intermolecular H-H bonding replaces non-directional induced dipole-induced

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dipole interaction in these alkane complexes. Newer insights from all these studies have come

calculations and modeling.

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not only from novel experimental techniques but also from the application of electronic structure

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Chiral molecules containing hydroxyl (-OH) and carboxylic (-COOH) moieties show strong O-H•••O hydrogen bonding [5]. On account of their wider importance in asymmetric synthesis and drugs, determination of absolute configuration and characterization of vibrational structure of hydrogen-bonded chiral molecules has assumed greater interest among spectroscopists [6]. In investigating the vibrational circular dichroism spectra of (S)-(-)-glycidol in water, Guochun Yang and Yunjie Xu have demonstrated that the hydrogen bonding network of (S)-(-)-glycidol – water clusters in solution phase makes the bonded water molecule optically active [7]. This optical activity of the water molecule, referred to as induced solvent chirality or chiral transfer, has been established due to intermolecular hydrogen bonding in different glycidol-water conformers near the water bending frequency region, 1650 cm-1. All this is manifested in the experimental vibrational absorption (VA, that is IR) and vibrational circular dichroism (VCD) spectra in the region 1700 – 1000 cm-1 strongly in favor of water subunits hydrogen bonded to glycidol molecules. The conformational and spectral analysis is performed by both molecular dynamics (MD) and DFT modeling calculations. Recently we reported the hydrogen-bonded vibrational spectrum of (R)-(-)-2-Pyrrolidinemethanol arising from its dimeric

ACCEPTED MANUSCRIPT species demonstrating its good agreement with the simulated B3LYP/6-311+G(d,p) spectra based on two possible intermolecular O-H•••O and N-H•••O bondings [8]. The IR absorption spectra of closely related molecules, succinic acid, formic acid and

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(S)-Phenylsuccinic acid share common features with that of (R)-(+)-Methylsuccinic acid. (R)(+)-Methylsuccinic acid (also known as (R)-(+)-Pyrotartaric acid) is reduced from (Z)-2-

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Methylbut-2-enedioic acid using enoate reductatse from Lycopersicum esculentum [9,10]. Its

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main applications include as organic building block in achiral synthesis [11]. The present paper is about the interpretation of hydrogen bonded vibrational spectra of (R)-(+)-Methylsuccinic

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acid, aided by B3LYP/6-311+G(d,p) level of modeling calculations. A strong –O-H stretching mode near 3048 cm-1(IR) accompanied by two mutually occurring vibrational modes, one at

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1700 cm-1 (IR) and one at 1651 cm-1 (Raman), dictated a hydrogen bonded dimer species locked with a local centre of symmetry as is true of molecules containing carboxylic acids [12-17]. We have shown that the observed spectrum is in good agreement with a simulated spectrum derived

Experimental:

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from six hydrogen bonded dimer species out of three computed stable monomers.

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The solid sample of MSA (C5H8O4) [18] was obtained from the Sigma Aldrich Co., and used as received. The Infrared spectrum was measured on Thermofisher Nicolet-6700 FTIR Spectrometer with a resolution 4 cm-1. The sample was illuminated using ETC Ever-Glo IR source in the spectrometer by forming a KBr pellet. The signals were collected by Deuterated Lanthium Triglycine Sulfate detector (DLaTGS). In order to achieve good signal-to-noise ratio 100 scans were accumulated and averaged. The Raman spectral measurements were made on a NXR 6700 FT-Raman spectrometer with LN2 cooled Ge detector. A diode-pumped air-cooled continuous wave Nd:YVO4 laser source with an excitation line at 1064 nm providing a power of ~500 mW. The spectra were measured with a total of 700 scans at resolution 4 cm-1.

Computational: Ab initio and density funtional theory (DFT) calculations have been carried out using Gaussian 09W and GaussView 5.0 suite of programs [19,20]. The levels for the calculation include RHF combined with 3-21G and B3LYP combined with 6-311+G(d,p) basis sets.

ACCEPTED MANUSCRIPT The molecular structure of (R)-(+)-Methylsuccinic acid is shown in Fig. 1. A rigid potential energy surface (PES) scan was performed at RHF/3-21G level to search for possible conformers. The dihedral angles say, τ1 and τ2 were varied simultaneously from 0º to 360º at an interval of

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10º. As a result three conformers were found. Further optimization of the conformers followed by harmonic frequency analysis were performed at B3LYP/6-311+G(d,p) level. These optimized

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structures have produced no imaginary frequency modes, proving that a true minimum on the

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potential energy surface for each of the conformers. The IR spectrum of (R)-(+)-Methylsuccinic acid has shown evidence of hydrogen bonding. Modelling for the inter-molecular dimer species

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formed out of the three computed monomers (conformers) is done on the same level. The results

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are discussed below.

Results and Discussion: Monomer and Dimer Analysis:

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The experimental IR and Raman spectra, as shown in Fig. 2(a) and (b), are characterized by band

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features not amenable to straightforward spectral interpretation. The IR region 3500 – 2500 cm-1 is dominated by broad structureless absorption with identifiable peaks at 3102 (shoulder) and

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3048 cm-1. Further, there is an intense and rather broad absorption band near 1700 cm-1; we tentatively identify a corresponding Raman band at 1651 cm-1. Other absorption bands are somewhat sharp but covered in broad envelope with one isolated broad band at 929 cm -1. The fact that the Raman band features do not correspond to the absorption bands and shows characteristic CH, CH3 and CC bands except a band at 1651 cm-1 suggests that the intermolecular influences are manifested more in the absorption spectrum. In the case of (R)-(-)-2Pyrrolidinemethanol we have seen a similar character of the vibrational spectra and a DFT hydrogen bonded dimer model satisfactorily accounted for it. Motivated by these results we propose that a similar donor –O-H from one monomer and acceptor -O=C from another monomer of (R)-(+)-Methylsuccinic acid form a hydrogen bonded dimer which would produce a vibrational spectrum in agreement with experiment. To probe possible dimer structures we also note that the widely separated IR band at 1700 cm-1 and the Raman band at 1650 cm-1 provide a hint of a structure typical of head-to-tail hydrogen bonded carboxylic acid groups with a local centre of symmetry [12-15].

ACCEPTED MANUSCRIPT To investigate the structure of dimer due to two monomers of (R)-(+)-Methylsuccinic acid as shown in Fig. 3, we performed a relaxed potential energy surface scan (PES) at RHF/3-21G level of theory to confirm the orientation of O-H in each of the two carboxylic

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groups by varying the dihedral angle C-C-O-H from 0° to 360° at 10° per interval one at a time. The PES produces a minimum for the dihedral angles C-C-O-H, 180° and the resultant structure

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is shown in Fig. 4(a). Fixing the two carboxylic moieties in this configuration the dihedral angles

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O8-C7-C1*-C3 (τ1) and O15-C14-C11-C1* (τ2) (where C* refers to the carbon atom at chiral centre in the molecule C5H8O4) were varied simultaneously from 0º to 360º at an interval of 10º.

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We found three conformers on the PES curves as shown in Fig 4(b). We further optimized the structures of the three conformers by performing harmonic frequency analysis at B3LYP/

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6-311+G(d,p) level by taking the PES results at RHF/3-21G level as input. The calculations yielded three conformers called C1, C2, and C3 that are shown in Fig. 5(a), (b) and (c). The C1 is the most stable conformer with a population of 79.5%; the second most stable conformer is C2

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with a population of 13.5%, being separated from C1 by 1.059 kcal/mol. The least populated

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conformer C3 with 7% is separated from C1 by 1.435 kcal/mol. In Table 1 we present the optimized geometrical parameters of all the three conformers C1, C2, and C3.

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In the next step we computed six dimer structures as -O-H•••O=C hydrogen bonded dimers from the three monomers shown in Fig. 3. The six dimers Di (i=1,2,3,4,5,6) with relative Gibbs free energies and populations are presented in Table 2. The most stable dimer D1 is the most populated at 38%. It is noted that while the dimers D2, D3 and D4 have nearly the same populations, the D5 and D6 are thinly populated. We have shown all the six dimer structures in Fig. 6 labelled with H•••O distances. The two H•••O distances vary slightly from dimer to dimer. The effect of this small variation on the centre of inversion is ignored. The angle between O-H and H•••O is almost linear (1790) and the H•••O distance, 1.686 Å, being shorter than the sums of their respective van der Waals radii viz., 2.72 Å, suggests a strong bonding [21,22]. As for the O•••O length, it is computed at 2.683 Å which is comparable to the value of 2.630 Å measured by B. Modec for Dimethylsuccinic acid and the value 2.660 Å for succinic acid crystal as reported by M. Suzuki et. al. [6,12].

Vibrational Analysis: For a good agreement between experimental and theoretical spectra, we have plotted Boltzmann population-weighted IR spectra of monomers and dimers and are

ACCEPTED MANUSCRIPT presented in Fig. 7 (a),(b),(c) with experimental IR spectrum; likewise, the Fig. 8 (a),(b) and (c) give for the Raman spectra. A complete assignment of all the observed bands with that of monomer and dimer are presented in Table 3. It is evident that the O-H stretching bands

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computed near 3615 cm-1 for a monomer in Fig. 7(c) shift to 3109 and 3020 cm-1 for the dimers in Fig. 7(b) agreeing with the observed bands at 3102 and 3048 cm-1 in Fig. 7(a). The monomer’s

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carbonyl band at 1732 cm-1 shifts to 1683 cm-1 as a dimer’s bonded carbonyl band agreeing well

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with the experimental band at 1700 cm-1. It is assigned to the asymmetric carbonyl stretching mode. The same holds good for Raman modes: the dimer’s bonded carbonyl band at 1637 cm-1

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in Fig 8(b) is correlated to the observed band at 1651 cm-1in Fig. 8(a) and is assigned to the symmetric carbonyl mode. In (R)-(+)-Methylsuccinic acid, there are two pairs of –O-H and – C=O groups, with only one pair of –O-H and –C=O takes part in the hydrogen bonding. The

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other ‘free’ pair gives rise to bands with frequencies too close to be resolved in the dimer spectrum. For example, two computed bands at 1742 and 1736 cm-1 are merged into a single

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band with another band at 1683 cm-1 correlated to the experimental IR band at 1700 cm-1. Hence

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we see two dimer bands corresponding to the single 1700 cm-1 band. The same modes are observed in the formic acid dimer as 1754, 1670 cm-1 and DFT predicted bands at 1765, 1691 1736 cm-1.

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cm-1 respectively [14]. In the present case, the non-bonded C=O modes are predicted at 1742 and

It is rather uncertain to ascertain whether or not the in-plane O-H bending mode near1400 cm-1 is characteristically increased to higher value because the region is covered with bands arising from coupled vibrations with CH2. However, there is one broad observed medium IR band at 929 cm-1 that is predicted at 908 cm-1 as a dimer mode and assigned to an out-of-plane O-H mode. The modes due to CH, CH2 and CH3 have remained uninfluenced by the hydrogen bonding and are fairly accurately computed. The region 3000 - 2800 cm-1 is marked by a series of CH, CH2 and CH3 stretching modes [23-26]. Predicted bands agree very well with these modes though not all are observed. A very strong IR mode at 2996 cm-1 is assigned to CH3 asymmetric stretching and is also computed at the same frequency. This mode is less discernible in the neighbourhood of the broad band due to bonded O-H stretching band but still is observed clearly. Its corresponding Raman band is a medium strong shoulder mode at 2990 cm-1. The symmetric stretch mode is observed at 2883 (IR) and 2897 (Raman)

cm-1. A computed band

2938 cm-1 is assigned as a CH2 symmetric stretching mode at 2945 (IR) and 2941 (Raman) cm-1.

ACCEPTED MANUSCRIPT Another strong IR band at 2928 cm-1 is assigned to CH stretching mode. The region 1500 – 1200 cm-1 shows several deformation modes arising from CH, CH2 and CH3 groups. The medium band 1463 cm-1 (IR) is correlated to 1450 cm-1 (DFT) mode and is assigned as CH3 deformation band.

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Another medium strong mode at 1423 cm-1 (IR) is due to CH2-COOH deformation. The computed 1364 cm-1 band is observed as weak bands both in IR and Raman at 1378 and 1370

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cm-1 respectively. It is a coupled vibration of CH2 and CH wag. Two modes due to bonded C-O-

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H deformation appear strongly in IR at 1317 and 1295 cm-1. And the free C-O-H deformation is observed near 1250 cm-1; it is strong as IR but weak as a Raman band at 1237 cm-1. Computed

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up-shifted bands as bonded deformation modes are at 1312 and 1303 cm-1 and free deformation mode is at 1247 cm-1. Coupled vibrations of skeletal stretching and deformation modes have

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appeared in 1200-1000 cm-1 region. A medium weak IR band at 1129 cm-1 is a coupled mode due to C-C and C-O stretching. Two modes are further computed and assigned to C-O and C-C-C stretch at 1097 and 1078 cm-1 appearing as weak IR bands; the skeletal deformation mode is

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identified at 1060 cm-1. Two modes due to bonded O-H wag or out of plane deformation are

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predicted at 960 and 908 cm-1. Only one mode is observed at 929 cm-1. It is a medium strong and relatively broad IR band. Below 900 cm-1, the free O-H out of plane deformation mode is

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assigned to only observed IR band at 630 and 594 cm-1. The skeletal and COOH deformation are weak IR bands observed at 826 and 804 cm-1. We may say that though the O-H•••O modes are localized in the dimer structure, the vibrational structure in IR spectrum is considerably modified with visible features. Further, we wish to point out that nowhere the explicit role of chirality of (R)-(+)-Methylsuccinic acid has been manifested in the IR nor Raman spectral features since these spectral features are not sensitive to chiral property. Still we might have computed a DFT chiral spectrum from the six dimers and correlated to experimental IR bands but such analysis would be redudant and outside the scope of the present study.

Conclusions: The DFT dimer model proposed with O-H•••O=C type of hydrogen bonding has been satisfactory in accounting for the experimental IR and Raman spectral band features of (R)-(+)Methylsuccinic acid. Three stable monomers have shown the possibility of six dimer structures with varying Boltzmann populations. It has been shown that all the dimers have a closed cyclic

ACCEPTED MANUSCRIPT geometry with a center of inversion and as a consequence, the experimental spectra are the result of contributions from the six different dimer spectra. Though the solid sample of (R)-(+)Methylsuccinic acid may have trimers or even polymers, the proposed dimer model at B3LYP/

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6-311+G(d,p) level of theory has been demonstrated to be satisfactory with accurate results. The computed geometrical parameters for the hydrogen bond lengths are reasonably accurate and are

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in favor of O-H•••O=C type of hydrogen bonding.

Acknowledgements:

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IR and Raman spectral measurements at USIC, Karnatak University is acknowledged with thanks. University Grants Commission, New Delhi, India, is acknowledged for the financial

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support through Major Research Project for implementing Gaussian 09, and for Junior Research Fellowship to YBC under RFSMS.

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[3] P. L. Geissler, Annu. Rev. Phys. Chem. 64 (2013) 317–337. [4] N. K. V. Monteiro, C. L. Firme, J. Phys. Chem. A 118 (9) (2014) 1730–1740 [5] F. Weinhold, R. A. Klein, Molecular Physics 110 (9–10) (2012) 565-579. [6] B. Modec, Crystals 3 (2013) 275-288. [7] Guochun Yang, Yunjie Xu, J. Chem. Phys., 130 (2009) 164506. [8] J. Tonannavar, Yashaswita B. Chavan, Jayashree Yenagi, Spectrochim Acta A 149 (2015) 860-868. [9] R. A. Sheldon, Chirotechnology: Industrial synthesis of optically active compounds, Marcel Dekkar. Inc, (1993). [10] H. Hiemstra (Vol. Editor), Science of Synthesis (Alkanes) (Houben-Weyl: Methods of Molecular Transformations, 48), Georg Thieme Verlag K. G., 2003. [11] www.sigmaaldrich.com [12] M. Suzuki, T. Shimanouchi, J. Mol. Spectro. 28 (1968) 394-410. [13] M. Suzuki, T. Shimanouchi, J. Mol. Spectro. 29 (1969) 415-425. [14] G. M. Florio, T. S. Zwier, E. M. Myshakin, K. D. Jordana, E. L. Sibert III, J. Chem. Phys., 118(4), (2003) 1735-1746. [15] D. Sajan, V. B. Jothy, T. Kuruvilla, H. Joe, J. Chem. Sci. 107(4) (2010) 511-519. [16] X. Gu and C. Q. Yang, Res. Chem. Intermed. 24(9) (1998) 979-996.

ACCEPTED MANUSCRIPT [17] V. Humblot, M. O. Lorenzo, C. J. Baddeley, S. Haq, R. Raval, J. Am. Chem. Soc. 126 (2004) 6460-6469. [18] Beil. 2, 111 (1964).

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[21] G. Desiraju, T. Steiner, The Weak Hydrogen Bond: In Structural Chemistry and Biology, Oxford Univ. Press, Inc., New York, 2001.

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[22] P. Schuster, G. Zundel, C. Sandorfy, The Hydrogen Bond: Recent developments in theory and experiments II Structure and Spectroscopy, North-Holland Publishing Company, Amsterdam, New York, Oxford, 1976. [23] A. E. Portyanskii, Y. T. Tashpulatov, S. I. Mekhtiev, V. M. Mamedova, J. Appl. Spectrosc. 13(5) (1970) 857862.

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[24] H. Baranska, J. K. Jaworska, R. Szostak, A. Romaniewska, J. Raman Spectrosc. 34 (2003) 68-76.

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[25] N. B. Colthup, L. H. Daly, S. E. Wiberley, Introduction to Infrared and Raman Spectroscopy, Academic Press, Inc., New York, 1964.

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[26] G. Socrates, Infrared Characteristic Group Frequencies, John Wiley, GB, 1980. [27] A.P. Scott, L. Random, J. Phys. Chem. 100 (1996) 16502–16513. [28] P. Van der Sluis, J. Kroon, Acta Cryst. C41 (1985) 956-959.

ACCEPTED MANUSCRIPT Figure Captions Fig.1. Molecular structure of (R)-(+)-Methylsuccinic acid.

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Fig.2. Experimental IR (a) and Raman (b) spectra of (R)-(+)-Methylsuccinic acid.

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Fig.3. A dimeric species formed of two monomers of (R)-(+)-Methylsuccinic acid showing a local centre of inversion.

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Note: R= Monomeric C4H7O2

Fig.4(a). PES scan curve showing a minimum for dihedral angle C-C-O-H=180˚ computed at

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RHF/3-21G level and a corresponding primary optimized structure of (R)-(+)-Methylsuccinic acid.

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Fig.4(b). PES scan curves for dihedral angles O8-C7-C1*-C3 and O15-C14-C11-C1* yielding the most stable conformers C1, C2 and C3.

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Fig.5. Conformers of (R)-(+)-Methylsuccinic acid computed at B3LYP/6-311+G(d,p) level.

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H•••O bonding.

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Fig.6. Optimized dimer structures of (R)-(+)-Methylsuccinic acid showing inter-molecular O–

Fig.7. Experimental (a), simulated dimer (b) and simulated monomer (c) IR spectrum of (R)-(+)Methylsuccinic acid.

Note: Break position shown in (b) is 20% of the axis length. Fig. 8. Experimental (a), simulated dimer (b) and simulated monomer (c) Raman spectrum of (R)-(+)-Methylsuccinic acid.

Note: Break position shown in (b) is 60% of the axis length.

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Figure 1

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R= Monomeric C4H7O2

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Figure 4(a)

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Figure 4(b)

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(a): C1

(b): C2 O8-C7-C1-C3= 116˚ O15-C14-C11-C1= -127˚ Population = 13.43%

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O8-C7-C1-C3= 119˚ O15-C14-C11-C1= 15˚ Population = 79.43%

(c): C3 O8-C7-C1-C3= 0˚ O15-C14-C11-C1= 13˚ Population = 7.14%

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Figure 6

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(b) Dimer

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ACCEPTED MANUSCRIPT Figure 8.

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ACCEPTED MANUSCRIPT Table Captions Table 1. Optimized geometrical parameters of the conformers C1, C2, C3.

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Note: *Experimental bond lengths and bond angles of Succinic acid [12] and (±) Malic acid [28].

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Table 2. Relative Gibbs free energy and Boltzmann populations of six dimer species. Table 3. Computed vibrational frequencies of Monomer and Dimer at B3LYP/6-311+G(d,p)

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level, with experimental IR and Raman frequencies and assignments of (R)-(+)-Methylsuccinic acid.

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Note: *Frequencies (in cm-1) are scaled with the factor 0.9613 [27] and Boltzmann population weighted, br=broad, sh=shoulder, s=strong, vs=very strong, ms=medium strong, w=weak,

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vw=very weak, mw=medium weak, sym=symmetric, asym=asymmetric.

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ACCEPTED MANUSCRIPT Table 1

Bond Length (Ǻ)

Conformers

C3

C1-H2

1.095

1.094

1.096

C1-C3

1.542

1.542

1.530

C1-C7

1.518

1.519

C1-C11

1.531

1.538

C3-H4

1.094

1.094

C11-H12/H13

1.093

1.094

C7-O8/ C14-O15

1.206

C7-O9/ C14-O16

1.356

O9-H10/ O16-H17

0.969

1.110

0.940

-

-

1.520

1.485

1.518

1.539

1.533

1.529

1.093

-

-

1.094

1.110

0.940

1.206

1.203

1.252

1.208

1.356

1.359

1.322

1.312

0.969

0.969

1.050

0.990

108.2

109.2

-

-

110.9

110.9

-

-

108.4

108.4

107.4

-

-

106.6

106.4

106.0

109.0

107.0

107.7

C1-C3-H4

110.9

AC CE P

H2-C1-C7

TE

H4-C3-H5

NU

D

H2-C1-C3

MA

Bond Angle (deg)

RI

PT

C2

(±) Malic Acid*

SC

C1

Succinic Acid*

H2-C1-C11

108.8

109.2

106.7

109.0

105.0

C1-C11-C14

112.9

113.5

112.8

109.0

109.0

C1-C7-O8/ C11-C14-O15

126.2

126.2

126.1

120.0

122.2

C1-C7-O9/ C11-C14-O16

111.6

111.5

111.7

120.0

112.1

O8-C7-O9/ O15-C14-O16

122.2

122.3

122.1

120.0

122.9

C7-O9-H10/ C14-O16-H17

107.2

107.2

106.4

112.0

114.0

Dihedral Angle (deg) O8-C7-C1-C3

119

116

0

-

-

O15-C14-C11-C1

15.66

-127.5

13.92

-

-

21

ACCEPTED MANUSCRIPT Table 2.

Monomer

Relative Gibbs free energies (kcal/mol)

D1

C1 with C1

0.00

D2

C1 with C3

0.38

D3

C2 with C3

0.54

D4

C3 with C3

0.64

D5

C1 with C2

0.87

08.79

D6

C2 with C2

1.23

04.81

38.10

SC

RI

19.96 15.31 13.01

AC CE P

TE

D

MA

NU

Population (%)

PT

Dimer

22

ACCEPTED MANUSCRIPT Table 3:

2945 vs,sh

2941 vs

MA D

2928 vs

SC

2990 ms,sh

NU

2996 vs

TE

2897 ms

1700 vs

1651 m 1464 w

1378 w

1370 w

1317 s,sh 1295 s

1305 w

1463 m

1423 ms

1419 w

1400 sh

1250 s 1225 ms

OH stretching (free) OH stretching (free) bonded OH asym stretch bonded OH sym stretch CH3 (Methyl) asym stretch CH3 (Methyl) asym stretch CH3 (Methyl) asym stretch CH3 (Methyl) asym stretch CH2 (Methylene) asym stretch CH2 (Methylene) asym stretch CH2 (Methylene) sym stretch CH2 (Methylene) sym stretch CH str, CH2 (Methylene) sym stretch CH str, CH2 (Methylene) sym stretch CH3 (Methyl) sym stretch CH3 (Methyl) sym stretch C=O stretch C=O stretch C=O asym stretch (bonded) C=O sym stretch (bonded) CH3 (Methyl) deformation CH3 (Methyl) deformation CH2 (Methylene) deformation CH2 (Methylene) deformation CH2-COOH deformation (bonded) CH2 (Methylene) deformation CH2 (Methylene) deformation CH2 (Methylene) deformation CH2 (Methylene) + CH wag CH3 (Methyl) wag CH3 (Methyl) wag CH2 (Methylene) + CH wag C-O-H deformation (bonded) C-O-H deformation (bonded) CH wag CH wag C-O-H deformation (free) C-O-H deformation (free) CH2 twist, CH wag

RI

3102 sh 3048 vs

2883 vs

Assignments

PT

Observed Freq* IR Raman

AC CE P

Computed Freq* Monomer Dimer 3615 3615 3611 3611 3109 3020 3006 3005 3003 2996 2986 2989 2977 2971 2972 2938 2936 2935 2932 2933 2930 2920 2917 2918 1738 1742 1732 1736 1683 1637 1451 1450 1444 1448 1442 1439 1419 1409 1402 1404 1395 1367 1364 1358 1361 1349 1340 1310 1312 1303 1285 1281 1274 1254 1256 1247 1225 1221

1237 w

23

ACCEPTED MANUSCRIPT

912

863 848 799 718 635 628 581 557 526 497

347

1041 w 942 w

929 ms 890 sh

849 w 826 w 804 vw

PT

RI

1083 w 1069 w

SC

1097 w 1078 w 1060 w

CH2 twist, CH wag CH2 twist CH2 twist CC stretch, C-O stretch CC stretch, C-O stretch C-O stretch, C-C-C stretch C-O stretch, C-C-C stretch C-C-C skeletal deformation C-C-C skeletal deformation CH3 rock CH3 rock OH wag (bonded) C-C-C skeletal stretch + CH3 rock C-C-C skeletal stretch + CH3 rock OH wag (bonded) C-C-C skeletal stretch CH2 (methylene) rock CH2 (methylene) rock C-C-C skeletal stretch C-C-C stretch +COOH deformation C-C-C stretch +COOH deformation C-C-C out of plane deformation C-C-C out of plane deformation COOH deformation (bonded) COOH deformation (bonded) OH out of plane deformation (free) OH out of plane deformation (free) Skeletal stretch OH out of plane deformation (free) OH out of plane deformation (free) C-C-O + C-C=O deformation CH2 (methylene) rock OH out of plane deformation (free) COOH rock (bonded) COOH rock (bonded)

NU

1026

1120 vw

901 w

MA

1059

1129 mw

849 m

D

1103 1078

1202 s

TE

1130

1214 1197 1192 1129 1113 1096 1079 1066 1053 1031 1025 960 919 912 908 873 867 860 850 816 803 720 715 669 652 629 597 586 576 565 538 507 481 412 388

754 vw

AC CE P

1207

570 vw

440 w 406 vw

393 m

682 vw

695 m

630 m 594 w

568 vw,sh 545 vw

24

ACCEPTED MANUSCRIPT Graphical abstract A study of hydrogen bonded vibrational spectra of (R)-(+)-Methylsuccinic acid, as aided by DFT dimer analysis

RI

IR

-1

Raman

MA 3600

3200

2800

AC CE P

TE

4000

-1

1700 cm

D

Raman Intensity

3048 cm

NU

O-H stretch

SC

Transmittance

PT

J. Tonannavar*, Yashaswita B. Chavan and Jayashree Yenagi

C=O stretch -1

1651 cm

2400 2000 1600 -1 Wavenumber (cm )

1200

800

400

•i

The vibrational band structures near 3100 – 3040 cm-1 in the IR and near 1650 cm-1 in the Raman spectra have shown the signatures of an inter-molecular hydrogen bonding. A DFT dimer model has been proposed on the basis of O-H•••O=C type of hydrogen bonding. The proposed dimer model at B3LYP/6-311+G(d,p) level of theory has yielded a total of six dimer structures with the Boltzmann population of 38% for the most stable dimer and 62% for the remaining five dimer populations. All the dimers form a closed cyclic structure with a local center of inversion linked by the two pairs of -O-H and -O=C. A Boltzmann population weighted vibration spectrum has, among others, given rise to one symmetric mode at 1683 and one asymmetric –C=O mode at 1637 cm-1 in agreement with the mutually exclusive experimentally observed IR band at 1700 and a Raman band at 1651 cm-1. The hydrogen bond length, H•••O, for the most stable dimer is 1.686 Å, shorter than the sums of van der Waals radii, 2.72 Å and the angle between O-H and H•••O is almost linear (1790), suggesting a strong hydrogen bonding. 25

ACCEPTED MANUSCRIPT Highlights Vibrational IR and Raman modes near suggest inter-molecular hydrogen bonding.



A DFT dimer model is proposed on the basis of O-H•••O=C type of hydrogen bonding.



The O-H and C=O vibrational stretching and bending modes are accurately predicted.

AC CE P

TE

D

MA

NU

SC

RI

PT



26