The "Lego brick" model A. Melli, F. Tonolo, V. Barone, C. Puzzarini, J. Phys. Chem. A 125, 9904 (2021)s based on the idea that a molecular system can be seen as formed by different fragments ("Lego bricks"). If accurate semi-experimental equilibrium geometries are available for these fragments, then the template molecule approach M. Piccardo, E. Penocchio, C. Puzzarini, M. Biczysko, V. Barone, J. Phys. Chem. A 119, 2058 (2015)an be used to account for the modifications occurring when going from the isolated fragment to the molecular system under investigation. The linear regression model G. Ceselin, V. Barone, N. Tasinato, J. Chem. Theory Comput. 17, 7290 (2021)an be employed to correct the linkage between the different fragments. The application of the "Lego brick" approach to substituted (mainly CN and CCH) benzenes and small PAHs will be presented. Rotational constants will be used to test the accuracy of these structures. A. Melli, F. Tonolo, V. Barone, C. Puzzarini, J. Phys. Chem. A 125, 9904 (2021)i M. Piccardo, E. Penocchio, C. Puzzarini, M. Biczysko, V. Barone, J. Phys. Chem. A 119, 2058 (2015)c G. Ceselin, V. Barone, N. Tasinato, J. Chem. Theory Comput. 17, 7290 (2021)c

It has long been recognized that the Beer-Lambert law allows the determination of molecular concentrations from experimentally measured transition intensities of absorption spectra provided the absorption cross section and path length are known, with the latter typically easily measured experimentally. For non-reactive molecules, a suitable standard of known concentration/pressure can provide the cross section if it is not already available from published molecular line lists. However, for many chemical intermediates, their reactivity precludes preparing such a standard and accounts for their frequent absence in molecular line lists. Nonetheless, such molecules play important roles in chemical reactions of significance both economically and environmentally; hence spectroscopic measurements of their concentrations can be very valuable. Historically such cross sections have been determined empirically by “on-the-fly” concentration measurements, mostly depending on the sometimes questionable assumption that the chemistry of these intermediates is thoroughly understood. For example, if the self-reaction rate constant of an isolated intermediate is accurately known, measurement of its temporal decay yields its concentration, thereby providing the line's cross section from its intensity measurement. Alternatively, if a reaction mechanism is assumed that determines the stoichiometry of a reactive intermediate and a non-reactive byproduct with a known cross section, the measurement of their relative transition intensities provides the cross section of the intermediate. While such empirical cross sections have proven quite useful, it is worthwhile to note that it is also possible to calculate cross sections for individual rovibronic transitions using a combination of quantum chemistry methods and spectral simulation software. The peroxy radicals, RO₂ (R=H or alkyl group), are important intermediates in combustion and tropospheric chemistry with considerable work reported on their chemistry and spectroscopy. For the peroxy radicals, we compare results obtained from empirical cross sections measured by cavity ringdown spectroscopy with theoretically calculated values using the CFOUR quantum chemistry suite and the PGOPHER spectral simulation software.

Derivation of structural, energetic, and spectroscopic properties for amino acids and polypeptide conformers from highly accurate theoretical approaches or mixed experimental-theoretical ones allows to set-up a database consistent with experiments https://github.com/VibESLab/AAP_SES_DataBase. In this work, we present works towards the set-up of database, which will extend over purely computational databases available so far. Both semi-experimental and accurate theoretical data make a reliable reference and their combination allows an extensive benchmark exploration of DFT methodologies. In fact, the application of DFT for specific properties usually requires careful benchmarking, but the databases available in literature usually tend to focus on the DFT performance for the computation of atomic and molecular energies rather than on structural and spectroscopic parameters. In this work we focus on the structural and spectroscopic properties as well as conformational energies suggesting that desired accuracy can be obtained by means of dispersion-corrected double hybrid functionals (DHF). 1. P. Wang, C. Shu, H. Ye, M. Biczysko, J. Phys. Chem. A 125, 45, 9826-9837, 2021 2. M. Sheng, F. Silvestrini, M. Biczysko, C. Puzzarini, J. Phys. Chem. A 125, 41, 9099–9114, 2021 3. C. Shu, Z. Jiang, M. Biczysko, Journal of Molecular Modeling, 26(6), 129, 2020 4. Z. Jiang, M. Biczysko, N. W. Moriarty, Proteins: Structure, Function, and Bioinformatics, 86, 273-278, 2018 5. C. Puzzarini, M. Biczysko, V. Barone, L. Largo, I. Pena, C. Cabezas, J.L. Alonso, J. Phys. Chem. Lett. 5, 534-540, 2014 6. V. Barone, M. Biczysko, J. Bloino, C. Puzzarini, Phys. Chem. Chem. Phys 15, 10094-10111, 2013

Constitutional isomers of MgC₄H elemental composition are theoretically investigated in this work using density functional theory at various levels. The linear doublet (²Σ⁺) isomer, 1-magnesapent-2,4-diyn-1-yl (1), was identified not only in the laboratory but also in the evolved carbon star, IRC+10216. While zero-point vibrational energy corrected relative energies obtained at the ROB3LYP/6-311++G(2d,2p) and ROωB97XD/6-311++G(2d,2p) levels correctly predict that isomer 1 is the global minimum geometry, the same obtained with UHF wavefunctions predict that a cyclic isomer, 1-ethynyl-1-λ³-magnesacycloprop-2-yne (2), is the global minimum structure. r0pt Figure Further electronic structure calculations carried out with GGA-functional, such as PBE0-D3 with def-TZVP basis set predict that isomer 1 is the global minimum irrespective of whether the solutions are based on ROHF or UHF wavefunctions. It is outlined here that the issue of spin-contamination (though under 5 %) becomes a serious one affecting the relative energies dramatically while using hybrid functionals such as B3LYP and ωB97XD in this case.

Molecular hydrogen, in the view of its simplicity, is well suited for testing quantum electrodynamics (QED) for molecules as well as for searching for new physics beyond the standard model such as new forces or extra dimensions. Furthermore, molecular hydrogen possesses a wide structure of ultranarrow rovibrational transitions with different sensitivities to the proton charge radius and proton-to-electron mass ratio. Therefore, the recent large progress in both theoretical and experimental determinations of the rovibrational splitting in different isotopologues of molecular hydrogen makes it a promising system for determination of some physical constants. We present the most accurate measurement of the position of the weak quadrupole S(2) 2–0 line in molecular deuterium. We have collected the spectra with a frequency-stabilized cavity ring-down spectrometer (FS-CRDS) with an ultrahigh finesse optical cavity (F = 637 000), optical frequency comb and operating in the frequency-agile, rapid scanning spectroscopy (FARS) mode. To reduce the systematic errors in line position determination, we analyzed our spectra with one of the most physically justified line-shape model describing the collisional effects, the speed-dependent billiard-ball profile (SDBBP), parameters of which are derived from ab initio quantum-scattering calculations. Despite working in the Doppler-limited regime, we reached 40 kHz of statistical uncertainty and 161 kHz of absolute accuracy. The accuracy of our measurement corresponds to the fifth significant digit of the leading term in QED correction. We were also able to test two other higher-order QED terms. We observed 2.3σ discrepancy with the recent theoretical value.

Resonant inelastic x-ray scattering (RIXS) provides valuable information on the electronic structure of molecules and materials that is not easily accessible by one-photon spectroscopies due to selection rule restrictions. With the continuing development in light source technologies, RIXS is rapidly becoming an important technique for the study of gas- and solution-phase molecular systems, and the need for reliable and inexpensive electronic structure methods to aid in the prediction and interpretation of complicated spectral features is becoming apparent. In this work, we present a simplified approach based on the linear-response time-dependent density functional theory (LR-TDDFT) formalism to simulate RIXS in 4d transition metal complexes without the need to solve the costly TDDFT quadratic-response equations. As an illustrative example, we simulate the 2p4d RIXS maps of three representative ruthenium complexes. The method is able to capture all experimental features in all three complexes with relative energies correct to within 0.6 eV, and at the cost of roughly two independent LR-TDDFT calculations.

Noncovalent interactions between metal ions and complex molecules play a central role in condensed matter systems from enzyme catalysis to membrane permeability. Vibrational probes are used to gain information on the local electrostatic environment during such condensed phase phenomena. While this approach has proven successful, there are shortcomings that have not been addressed. In the condensed phase, the intrinsic molecular level response (polarization, isomerization, etc.) is obscured by the effects of hydrogen bonding, solvent exchange, and other external factors such as heterogeneity in the medium. Cryogenic ion vibrational predissociation spectroscopy (CIVP) is a useful way to obtain the detailed information needed to unambiguously measure the intrinsic response of vibrational electric field probes. In particular, this paper describes how CIVP can be used to quantify the separate roles of proximal molecular ions and interactions with solvent. We apply this approach to monitor the responses of palmitic acid and benzo-15-crown-5-nitrile to various metals in the +1 and +2 charge states. Both of these species contain popular vibrational probes (carboxylate in the former and nitrile in the latter), and are relevant, molecular-level models of scaffolds present in biological systems. Vibrational spectra of the cold ( 20K), H2 or He tagged ions are collected and assignments are made with the assistance of electronic structure calculations. This technique provides a new window into the intrinsic response of vibrational probes that are widely used in biophysical applications.

Vibrational spectra of small molecules effectively probe the underlying potential energy hypersurface, which can be tested when combined with accurate anharmonic calculations.P. R. Franke, J. F. Stanton, G. E. Douberly, J. Phys. Chem. A 2021, 125, 1301-1324; J. M. Bowman, T. Carrington, H.-D. Meyer, Mol. Phys. 2008, 106, 2145-2182.articularly suited for a performance test of quantum chemical gas phase calculations are spectra recorded in a supersonic expansion, as significant rotational cooling is achieved while the molecules or molecular clusters remain isolated in the gas phase. One of the smallest reference systems for such a benchmarking study is the formic acid monomer with its cis-trans-torsional isomerism.D. P. Tew, W. Mizukami, J. Phys. Chem. A 2016, 120, 9815-9828; F. Richter, P. Carbonnière, J. Chem. Phys. 2018, 148, 064303; A. Nejad, E. L. Sibert III, J. Chem. Phys. 2021, 154, 064301.ecently, new vibrational reference data on the stretching vibrations of all four H/D isotopologues of the higher-energy cis-conformer were provided via the combination of Raman jet spectroscopy with thermal excitation.A. Nejad , M. A. Suhm, K. A. E. Meyer, Phys. Chem. Chem. Phys. 2020, 22, 25492-25501.nother very interesting carboxylic acid for such a study is the smallest acetylenic acid, HCC-COOH, whose cis- and trans-rotamers will be discussed in this contribution. Of particular interest are two almost isoenergetic trans-fundamentals of different symmetry which are shown to be a particularly useful benchmarking target, but also the dimers of the CC-stretched formic acid prove to be an insightful reference system for benchmarking.K. A. E. Meyer, A. Nejad, Phys. Chem. Chem. Phys. 2021, 23, 17208-17223.html:<hr /><h3>Footnotes: P. R. Franke, J. F. Stanton, G. E. Douberly, J. Phys. Chem. A 2021, 125, 1301-1324; J. M. Bowman, T. Carrington, H.-D. Meyer, Mol. Phys. 2008, 106, 2145-2182.P D. P. Tew, W. Mizukami, J. Phys. Chem. A 2016, 120, 9815-9828; F. Richter, P. Carbonnière, J. Chem. Phys. 2018, 148, 064303; A. Nejad, E. L. Sibert III, J. Chem. Phys. 2021, 154, 064301.R A. Nejad , M. A. Suhm, K. A. E. Meyer, Phys. Chem. Chem. Phys. 2020, 22, 25492-25501.A K. A. E. Meyer, A. Nejad, Phys. Chem. Chem. Phys. 2021, 23, 17208-17223.

r0pt Figure The vibrational spectroscopy of formic acid, F, has seen new and important experimental and theoretical impulses in the past six years.(exp) K. Hull, T. Wells, B. E. Billinghurst, H. Bunn, P. L. Raston, AIP Adv. 2019, 9, 015021; K. A. E. Meyer, M. A. Suhm, Chem. Sci. 2019, 10, 6285; A. Nejad, M. A. Suhm, K. A. E. Meyer, Phys. Chem. Chem. Phys. 2020, 22, 25492. (theo) D. P. Tew, W. Mizukami, J. Phys. Chem. A 2016, 120, 9815; F. Richter, P. Carbonnière, J. Chem. Phys. 2018, 148, 064303; A. Aerts, P. Carbonnière, F. Richter, A. Brown, J. Chem. Phys. 2020, 152, 024305; A. Nejad, E. L. Sibert III, J. Chem. Phys. 2021, 154, 064301.n a combined experimental and theoretical approach, the vibrational database of F below 4000 cm⁻¹ is reviewed and extended to 189 band centres [ ∼ 300% increase], including a plethora of highly-excited vibrational states, both torsional conformers, and several isotopologues [¹H, ²H, ¹²C, ¹³C, and ¹⁶O].A. Nejad, PhD thesis, submitted (2022).ssentially, the vibrational characterisation of its skeletal modes below 3500 cm⁻¹ can be regarded as complete which is also an important stepping stone in understanding the complex vibrational dynamics of its cyclic dimer.A. Nejad, K. A. E. Meyer, F. Kollipost, Z. Xue, M. A. Suhm , J. Chem. Phys. 2021, 155, 224301 new key insight is that the impact of the OH bend-torsion resonance [ν₅ and 2ν₉] on the entire vibrational dynamics of trans-HCOOH is more far-reaching than previously believed. Beyond 3500 cm⁻¹, which is also near the expected trans→cis isomerisation threshold, this resonance polyad is indicated to play an important role in the perturbations of the OH stretching fundamental [ν₁]. In this contribution, new spectroscopic developments are summarised and promising future research directions for F are discussed. In particular, its importance and suitability for the purpose of benchmarking electronic and nuclear vibrational methodologies are highlighted.

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Footnotes:

(exp) K. Hull, T. Wells, B. E. Billinghurst, H. Bunn, P. L. Raston, AIP Adv. 2019, 9, 015021; K. A. E. Meyer, M. A. Suhm, Chem. Sci. 2019, 10, 6285; A. Nejad, M. A. Suhm, K. A. E. Meyer, Phys. Chem. Chem. Phys. 2020, 22, 25492. (theo) D. P. Tew, W. Mizukami, J. Phys. Chem. A 2016, 120, 9815; F. Richter, P. Carbonnière, J. Chem. Phys. 2018, 148, 064303; A. Aerts, P. Carbonnière, F. Richter, A. Brown, J. Chem. Phys. 2020, 152, 024305; A. Nejad, E. L. Sibert III, J. Chem. Phys. 2021, 154, 064301.I A. Nejad, PhD thesis, submitted (2022).E A. Nejad, K. A. E. Meyer, F. Kollipost, Z. Xue, M. A. Suhm , J. Chem. Phys. 2021, 155, 224301A