PCCP View Article Online PAPER View Journal | View Issue Predicting OH stretching fundamental wavenumbers of alcohols for conformational Cite this: Phys. Chem. Chem. Phys., 2021, 23, 5629 assignment: different correction patterns for density functional and wave-function-based methods† Robert Medel * and Martin A. Suhm A model is presented for the prediction of OH stretching fundamental wavenumbers of alcohol conformers in the gas phase by application of a small set of empirical anharmonicity corrections to calculations in the harmonic approximation. In contrast to the popular application of a uniform scaling factor, the local chemical structure of the alcohol is taken into account to greatly improve accuracy. Interestingly, different correction patterns emerge for results of hybrid density functional (B3LYP-D3 and PBE0-D3) and wave-function-based Creative Commons Attribution 3.0 Unported Licence. methods (SCS-LMP2, LCCSD(T*)-F12a and CCSD(T)-F12a 1D). This raises questions about electronic structure deficiencies in these methods and differences in anharmonicity between alcohols. After its initial construction on the basis of literature assignments the model is tested with Raman jet spectroscopy of propargyl alcohol, Received 24th January 2021, cyclohexanol, borneol, isopinocampheol and 2-methylbutan-2-ol. For propargyl alcohol a spectral splitting Accepted 24th February 2021 attributed to tunneling is resolved. PBE0-D3 is identified as a well performing and broadly affordable À1 DOI: 10.1039/d1cp00342a electronic structure method for this model. A mean absolute error of 1.3 cm and a maximum absolute error of 3 cmÀ1 result for 46 conformers of 24 alcohols in a 60 cmÀ1 range, when a single parameter is rsc.li/pccp adjusted separately for each alcohol substitution class(methanol,primary,secondary,tertiary). This article is licensed under a 1 Introduction the hydroxy group and can thus be used as a probe for the constitution and conformation of the alcohol as well as for Open Access Article. Published on 25 February 2021. Downloaded 9/28/2021 9:16:54 PM. Alcohols, compounds with a hydroxy group –OH bound to a possible inter- or intramolecular hydrogen bonding. In this work 1 saturated carbon atom R3C–, are among the most common classes we use Raman spectroscopy as our method of choice, because it of organic compounds. By torsion around the C–O-bond,2 and typically leads to narrow and intense rovibrational band maxima possibly other bonds in the molecular frame,3 most alcohols can and reveals the collisional cooling along the jet expansion with a adopt different conformations. To comprehend the details of narrow laser focus. alcohol aggregation4–6 and reactivity7,8 it is essential to under- Rotational structure, accessible by rotational9,10 or high stand conformational isomerism. Such an understanding profits resolution electronic11,12 spectroscopy, is also very powerful in from isolated molecules at low temperatures, as they are accessible obtaining conformational information, but the global character in supersonic jet expansions. This helps to identify the most stable of rotational constants makes it increasingly difficult to determine conformers, reduces spectral complexity and facilitates compar- the position of the light hydroxy hydrogen, when the molecules ison with quantum chemical predictions. The hydroxy group is a increase in size.13,14 In such situations, the synergy between chromophore for vibrational spectroscopy, as the OH stretching different methods is most powerful to reach unambiguous fundamental usually carries substantial infrared and Raman activity. assignments.13,15 Furthermore its frequency is sensitive to the local environment of Another valuable assignment synergy is provided by the com- parison with quantum chemical predictions, typically carried out in Institute of Physical Chemistry, University of Goettingen, Tammannstr. 6, 37077 the computationally efficient double-harmonic approximation Goettingen, Germany. E-mail: [email protected], [email protected] (linear restoring force and linear dipole change for Cartesian † Electronic supplementary information (ESI) available: More test cases, updated displacements). Deficiencies in the electronic structure method, correlation graphs and corrections, structures as well as experimental and calculated wavenumbers for all alcohols in the data set, sample inputs, tests basis set incompleteness and especially the neglect of anharmo- for hydrogen-bonded conformers, extended range spectra and more. See DOI: nicity, however, lead to a mismatch between experimental and 10.1039/d1cp00342a predicted values, for rotational and vibrational spectroscopy. This journal is the Owner Societies 2021 Phys. Chem. Chem. Phys.,2021,23,5629À5643 | 5629 View Article Online Paper PCCP Even if the predicted rotational constants are close or the 9and13cmÀ1. This might indicate the possibility of an empirical vibrational pattern is accurate, comparison to experiment can correction to close the remaining gap from neglected normal still lead to wrong assignments. In vibrational spectroscopy, mode off-diagonal anharmonicity. Recovering parts of it by large intensity differences between conformers or spectral overlap adding the couplings with the CO stretching and the COH between different monomer bands as well as with dimer signals bending mode (3D local mode model)26 removes the general are typical obstacles. overestimation, but the spread of the deviations becomes larger An example for this challenge is the initial assignment of with values between À6and+4cmÀ1, resulting in MAE = 2.6 cmÀ1 two predicted conformers for phenylmethanol to two observed and MAX = 6 cmÀ1. The CCSD(T) 1D and 3D models are currently bands,16,17 which was later corrected to one monomer and computationally affordable for alcohols with up to about six one dimer with the help of a scaling factor obtained from carbon atoms.27 One main disadvantage is the rather complicated 2-phenylethanol.13 Another challenging case is found in the manual procedure not easily conducted by the non-specialist, monoterpene alcohols menthol and neomenthol. A single especially for the 3D version. intense band was observed for each of the two constitutional To explore possible low-cost alternatives with routine normal isomers in the monomer region and theoretical methods mode calculations in the harmonic approximation, we compare differed in their prediction of the respective most stable confor- in this article the correlation between results of five quantum mation responsible for these signals.18 Apredictedcloseharmonic chemical methods and a training set of 35 alcohol conformers frequency match for two conformers across the two species helped, collated from literature supersonic jet assignments. The devised because it mirrored the experimental finding. The resulting model for each method is then tested with Raman jet spectra of assignment for menthol was later confirmed by rotational spectro- additional alcohols. scopy,10 for neomenthol it is still pending. These examples demonstrate that the prediction of accurate absolute values for experimental OH stretching fundamentals 2 Methods Creative Commons Attribution 3.0 Unported Licence. is desirable and that they might be obtained from inexpensive calculations when an empirical correction from observed tran- 2.1 Computational techniques sitions of other alcohols is applied. This was attempted already Density functional computations were carried out with the Gaus- 35 years ago using infrared spectra of vapors at room or sian 09 Rev. E.01 program package.28 The B3LYP29–32 and PBE033 elevated temperature and a reparameterized MM2/CNDO force functionals together with Grimmes D3 dispersion correction with field approach.19 Subsequent tests of the model on further two-body terms and Becke–Johnson damping34 were employed. alcohols, however, revealed double-digit cmÀ1 deviations,20,21 The minimally augmented may-cc-pVTZ basis set,35 the ultrafine which is of limited use in the light of conformers often differing integration grid, verytight convergence criteria and no density only in the single-digit range. More encouraging is a 2008 fitting were used. This article is licensed under a study22 on the prediction of (among others) free and weakly Wavefunction-based methods were carried out with the hydrogen-bonded OH stretching wavenumbers of biomolecules Molpro 2020.1 program package.36 Møller–Plesset perturbation under jet conditions. With scaling of harmonic B3LYP/6-311++G** theory in second order (MP2) was used in its local variant (L) with results a mean absolute error (MAE) of 2.8 cmÀ1 and a maximum spin-component-scaling (SCS),37 the fully augmented aug-cc-pVTZ Open Access Article. Published on 25 February 2021. Downloaded 9/28/2021 9:16:54 PM. absolute error (MAX) of 12 cmÀ1 for the correlation with experi- basis set and the program’s default fitting basis sets for density ment was achieved. fitting. Local coupled-cluster38,39 with full singles and doubles as An alternative to empirical or systematic23 corrections to well as scaled40 non-iterate perturbative triples was used with harmonic normal modes is the local mode approach, which explicit correlation in the F12a variant with the Molpro default includes diagonal anharmonicity (deviation from a parabolic geminal exponent40 of 1.0 and the VDZ-F12 basis set41,42 together potential) by construction.24 The reduced dimensionality with density fitting, which we will abbreviate as LCCSD(T*). For allows to employ a higher-level electronic structure method both wavefunction-based-methods