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WIREs Comput Mol Sci
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# The virtual multifrequency spectrometer: a new paradigm for spectroscopy

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Ongoing developments of hardware and software are changing computational spectroscopy from a strongly specialized research area to a general tool in the inventory of most researchers. Increased interactions between experimentally oriented users and theoretically oriented developers of new methods and models would result in more robust, flexible and reliable tools, and studies for the systems of increasing complexity, which are of current scientific and technological interest. This is the philosophy behind this review, which presents the development of a so‐called virtual multifrequency spectrometer (VMS) including state‐of‐the‐art approaches in a user‐friendly frame. The current status of the VMS tool will be illustrated by a number of case studies with special reference to infrared and UV–vis regions of the electromagnetic spectrum including also chiral spectroscopies. Only the basic theoretical background will be provided avoiding explicit equations as far as possible, and pointing out the most recent advancements beyond the standard rigid‐rotor harmonic‐oscillator model coupled to vertical electronic excitation energies. WIREs Comput Mol Sci 2016, 6:86–110. doi: 10.1002/wcms.1238

• Structure and Mechanism > Molecular Structures
• Theoretical and Physical Chemistry > Spectroscopy
The framework of the whole virtual multifrequency spectrometer (VMS) project.
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EPR spectrum of tempone (upper panel) and nuclear magnetic resonance (NMR) spectrum of benzoic acid dimer (lower panel). In the latter case, the total spectrum is plotted in black and the contributions of different 17O in various colors.
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Vibrational(torsional) energy levels and one‐dimensional vibrational wavefunctions of methanol superimposed to the potential energy profile along the HOCH dihedral angle.
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Theoretical one‐photon absorption (OPA) (upper panel) and resonance Raman (RR) (lower panel) spectra for the S 2S 0 transition of imidazole in water. All simulations have been carried out using the time‐dependent (TD) adiabatic Hessian/Franck–Condon (AH|FC) algorithm with normal modes built with Cartesian or internal coordinates. Electronic structure calculations have been carried out at the density functional theory (DFT) (S 0) and time‐dependent density functional theory (TD‐DFT) (S 2) level including bulk solvent contributions by means of the polarisable‐continuum model. Broadening effects have been included using Gaussian functions with half‐width at half‐maximum (HWHM) of 100 cm−1 (for the OPA spectrum) and Lorenztian functions with HWHM of 10 cm−1 (for the RR spectrum). The equilibrium geometries of both electronic states are also shown, along with the Duschinsky matrices ( J ) computed using Cartesian or internal coordinates. The elements of $J i j 2$ are calculated and a shade of gray is associated with each element (i,j) in the figure based on its absolute value (0, white; 1, black).
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Experimental and theoretical anharmonic difference time‐resolved infrared spectroscopy (TRIR) spectra of Rebpy. Anharmonic spectra have been calculated using the three‐mode reduced (RD3) and full‐dimensionality (FD) approaches.
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Contributions of various effects [bulk solvent (PCM), vibronic contributions (FC|VG) of intramolecular vibrations, and two explicit methanol molecules] to the overall absorption spectrum of chlorophyll a1 in the 250‐to 700‐nm energy range, as compared to the experimental data obtained in methanol solution, see Ref for details.
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Simulated UV–vis spectrum of chlorophyll‐a analyzed by virtual multifrequency spectrometer (VMS)‐draw: total spectrum obtained as the sum of separate electronic transitions, predicted color as perceived by a human eye, and normal mode visualization.
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Total simulated VG|FC spectrum of the 1,2Al‐PT9(H2O)4 alizarin complex, along with the single‐state contributions to the spectra band shape. See Ref for details.
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Theoretical and experimental spectra of isolated pyrimidine in the 3.5–9.5 eV energy range computed at the Franck–Condon/vertical gradient (FC|VG) level, applying the hybrid pHF/density functional theory (DFT) approaches (main panel, red). The S 1S 0 electronic transition along with the assignment of the most intense vibronic bands, computed at the FCHT|AH level, with anharmonic corrections though effective scaling procedure using the best CC/B3LYP estimates for the electronic ground state (inset, green). See Ref for details.
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Fully anharmonic infrared (IR), Raman, vibrational circular dichroism (VCD), and Raman optical activity (ROA) spectra of methyloxirane compared to their experimental counterparts measured in low‐temperature Ar Matrix (IR, VCD) or the gas phase (Raman, ROA). Vibrational wavenumbers have been computed at the ‘cheapCC’/B3LYP level in conjunction with B3LYP/SNSD intensities; all spectra have been convoluted by means of Lorentzian distribution functions with full‐width at half‐maximum 2 cm− 1.
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Theoretical and experimental infrared (IR) spectra of napthalene in the 300–3500 cm−1 range. Fully harmonic (Harm., harmonic wavenumbers, and intensities) spectra, scaled harmonic spectra (Scaled., scaled harmonic energies), spectra obtained by combining anharmonic wavenumbers with harmonic intensities (Anh. Freq.) and fully anharmonic spectra (Full Anh., anharmonic wavenumbers and intensities). All computations at the B3LYP/SNSD level, mean absolute error with respect to the experiment, see Ref for details.
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Performance of B3LYP/SNSD and hybrid B2PLYP/B3LYP and Coupled Cluster (CC) with Single Double and perturbative Triple (CCSD(T))/B3LYP models for the computation of anharmonic vibrational wavenumbers at the GVPT2 level. All anharmonic corrections have been computed at the B3LYP level with basis sets of at least double‐ζ plus polarization quality (mainly belonging to the SNSD/N07D family). Harmonic wavenumbers at the B2PLYP and CCSD(T) levels have been computed with basis sets of at least cc‐pVTZ quality. Mean absolute error with respect to experimental data for about 300 fundamental wavenumbers of small‐to‐medium‐size molecular systems; Ref : small molecules (H2O, NH2, and NH3), halo‐organic systems (halo‐methanes, halo‐ethylenes), acroleine, glycine (Ip, IIn, IIIp, and VIp conformers), phenyl, pyrimidine, uracil; Ref : oxiranes (oxirane, trans‐2,3‐dideuteriooxirane, and methyloxirane).
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General theoretical framework for vibrationally averaged properties together with vibrational and vibronic transitions along with quantum mechanical (QM) models for potential energy surface (PES) and property surface (PS) computations.
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Fully polarisable multiscale model: quantum mechanical (QM)/MM(FQ)/polarisable‐continuum model (PCM).
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