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Benchmarking Quantum Chemistry with Rotational Spectroscopy or Benchmarking Rotational Spectroscopy with Quantum Chemistry?
No full textQuantum chemistry has nowadays reached such an advanced level that highly accurate results can be achieved for energies and properties of small to medium-sized molecules. For these high-level calculations the requirements are efficient treatment of electron correlation via coupled-cluster theory, basis-set extrapolation techniques, incorporation of core correlation, relativistic as well as vibrational effects together with the use of suitable additivity schemes.
Nevertheless, despite all the progress made so far, it is still essential to benchmark the results from quantum-chemical calculations. Data from rotational spectroscopy are ideally suited for this purpose, as this technique provides, in particular for small molecules in the gas phase, highly accurate results. On the other hand, however, measurements and analyses of rotational spectra are not often straightforward. State-of-the-art quantumchemical computations are therefore needed to guide the investigation and in particolar to assist in the determination of the spectroscopic parameters of interest. Quantum chemistry in this way allows to verify (“benchmark”) results from rotational spectroscopy. A statistical analysis of the accuracy of theoretically predicted rotational constants will be presented as an example for the
benchmark of quantum chemistry via rotational spectroscopy [1]. On the other hand, the determination of the hyperfine parameters of dihalogencarbenes (CF2 and CCl2) will show the need of “benchmarking” results from experiments [2].
Based on all the considerations given above, the answer to the “title question” turns out to be not clear-cut. What we suggest instead is to exploit a fruitful interplay of theory (quantum chemistry) and experiment (rotational spectroscopy). The power of such an interplay will be demonstrated by a few examples [3,4].
1) C. Puzzarini, M. Heckert, J. Gauss, J. Chem. Phys., 2008, 128, 194108.
2) C. Puzzarini, S. Coriani, A. Rizzo, J. Gauss, Chem. Phys. Lett., 2005, 409, 118.
3) C. Puzzarini, G. Cazzoli, M.E. Harding, J. Vázquez, J. Gauss, J. Chem. Phys., 2009, 131, 234304.
4) S. Thorwirth, M. E. Harding, D. Muders, J. Gauss, J. Mol. Spectrosc., 2008, 251, 220; P. Botschwina, C. Puzzarini, J. Mol. Spectrosc., 2001, 208, 292; G. Cazzoli, L. Cludi, M. Contento, C. Puzzarini, J. Mol. Spectrosc., 2008, 251, 229
Hyperfine structure of rotational spectra: interplay of experiment and theory
No full textThe determination of (hyper)fine parameters such as quadrupole-coupling, spin-spin coupling, and spin-rotation constants is one of the aims of high-resolution rotational spectroscopy. These parameters are relevant not only from a spectroscopic point of view, but also from a physical and/or chemical viewpoint, as they might provide detailed information on the chemical bond, structure, etc. Furthermore, the hyperfine structure of rotational spectra is so characteristic that its analysis may help in assigning the spectra of unknown species [1].
However, the experimental determination of hyperfine constants can be a challenge not only for actual problems in resolving hyperfine structures themselves, but also due to the lack of reliable estimates or the complexity of the hyperfine structure itself. It is thus important to be
able to rely on good predictions for such parameters, which can nowadays be provided by quantum-chemical calculations [2]. In fact, the aim of this presentation is to show how fruitful the interplay between experiment and theory can be in this field. A number of examples will be presented to illustrate this interplay in the investigation of hyperfine structures of rotational spectra. Those include isotopic species of water and formic acid as well as heavy-element containing species, as CH2FI.
From an experimental point of view, we focus on the Lamb-dip technique. This technique allows to improve the resolving power in the millimeter- and submillimeter-wave frequency range by at least one order of magnitude, thus making it possible to perform sub-Doppler measurements as well as to resolve narrow hyperfine structures. In particular, the high resolution that can be achieved by our experimental set up will be demonstrated by a few representative examples [3,4].
Concerning theory, the theoretical background for the required quantum-chemical calculations will be briefly reviewed, and a particular emphasis on the computational requirements will be given [2]. It will be demonstrated that high-level calculations can provide very reliable values for hyperfine parameters (quadrupole coupling constants, spinrotation tensors, spin-spin couplings, etc.) and how theoretical predictions are often essential for a detailed analysis of the hyperfine structure of the recorded spectra [5].
[1] G. Cazzoli, C. Puzzarini, A. Gambi, J. Chem. Phys. 120 (2004) 6495-6501.
[2] C. Puzzarini, J. F. Stanton, J. Gauss, Int. Rev. Phys. Chem. 29 (2010) 273-367.
[3] G. Cazzoli, C. Puzzarini, J. Mol. Spectrosc. 226 (2004) 161-168.
[4] G. Cazzoli, L. Dore, C. Puzzarini, Astron. Astrophys. 507 (2009) 1707-1710.
[5] C. Puzzarini, G. Cazzoli, M. E. Harding, J. Vázquez, J. Gauss, J. Chem. Phys. 131 (2009) 234304/1-11
Quantum-chemical determination of Born-Oppenheimer breakdown parameters for rotational constants: the open-shell species CN, CO+ and BO
No full textThe quantum-chemical protocol for computing Born-Oppenheimer breakdown corrections to rotational constants in the case of diatomic molecules is extended to open-shell species. The deviation from the Born-Oppenheimer equilibrium rotational constant is obtained by considering three contributions: the adiabatic correction to the equilibrium bond distance, the electronic contribution to the moment of inertia requiring the computation of the rotational g-tensor, and the so-called Dunham correction. Values for the Born-Oppenheimer breakdown parameters of CN, CO+, and BO in their (2)sigma(+) electronic ground states are reported based on coupled-cluster calculations of the involved quantities and compared to available experimental data
Rotational spectra of isotopic species of silyl fluoride. Part II: theoretical and empirical equilibrium structure
No full textThe equilibrium structure of silyl fluoride, SiH3F, has been reinvestigated using both theoretical and experimental data. With respect to the former, quantum-chemical calculations at the coupled-cluster level have been employed together with extrapolation to the basis set limit, consideration of higher excitations in the cluster operator, and inclusion of core correlation as well as relativistic corrections (r(Si–F) = 1.5911 Å, r(Si–H) = 1.4695 Å, and FSiH = 108.30°). A semi-experimental equilibrium structure has been determined based on the available rotational constants for the various isotopic species of silyl fluoride (28SiH3F, 28SiD3F, 29SiH3F, 29SiD3F, 30SiH3F, 30SiD3F, 28SiH2DF, and 28SiHD2F) together with computed vibrational corrections to the rotational constants (r(Si–F) = 1.59048(6) Å, r(Si–H) = 1.46948(9) Å, and FSiH = 108.304(9)°)
"Fine- and hyperfine-structure of rotational spectra predicted by ab initio computations".
No full textVibrational corrections to dipolar coupling constants: an alternative for determining equilibrium distances from rotational spectroscopy.
No full textThe main interaction between the spins of two nuclei is the dipole-dipole coupling between their magnetic moments. The importance of these interactions lies in the facts that
a) dipolar coupling constants should be considered for the proper analysis of the hyperfine structure in rotational spectra and
b) that dipolar-coupling tensors provide an alternative way for determining bond distances (based either on the analysis of rotational or NMR spectra).
In principle, the determination of the dipolar coupling constants just require knowledge of the molecular equilibrium geometry, though quantum-chemical investigations are necessary to evaluate the vibrational corrections. In this presentation we will compare computed dipolar coupling constants for several small to medium-sized molecules to experiment, and demonstrate the importance of including vibrational corrections for accurate predictions. In addition, we will show how experimental dipolar coupling constants together with computed vibrational corrections can be used to derive equilibrium bond distances
Hyperfine structure of rotational spectra: state-of-the-art experimental and theoretical determinations
No full textThe determination of (hyper)fine parameters such as quadrupole-coupling, spin-spin coupling, and spin-rotation constants is one of the aims of high-resolution rotational spectroscopy. These parameters are relevant not only from a spectroscopic point of view, but also from a physical and/or chemical viewpoint, as they might provide detailed information on the chemical bond, structure, etc. In addition, the hyperfine structure of rotational spectra is so characteristic that its analysis may help in assigning the spectra of unknown species. Nevertheless, the experimental determination of hyperfine constants can be a challenge not only for actual problems in resolving hyperfine structures themselves, but also due to the lack of reliable estimates or the complexity of the hyperfine structure itself. It is thus important to be able to rely on good predictions for such parameters, which can nowadays be provided by quantum-chemical calculations. In fact, the aim of this presentation is to show how fruitful the interplay between experiment and theory can be in this field.
A number of examples will be presented to illustrate this interplay in the investigation of hyperfine structures of rotational spectra. Among others, those include isotopic species of water, bromofluoromethane, ammonia and hydrogen cyanide.
From an experimental point of view, we focus on the Lamb-dip technique. This technique allows to improve the resolving power in the millimeter- and submillimeter-wave frequency range by at least one order of magnitude, thus making it possible to perform sub-Doppler measurements as well as to resolve narrow hyperfine structures. In particular, the high resolution that can be achieved by our experimental set up will be demonstrated by a few representative examples.
Concerning theory, the theoretical background for the required quantum-chemical calculations will be briefly reviewed, and a particular emphasis on the computational requirements will be given. It will be demonstrated that high-level calculations can provide very reliable values for hyperfine parameters (quadrupole coupling constants, spin-rotation tensors, spin-spin couplings, etc.) and how theoretical predictions are often essential for a detailed analysis of the hyperfine structure of the recorded spectra
Rotational spectra of isotopic species of silyl fluoride. Part I: Lamb-dip measurements and quantum-chemical calculations
No full textThe Lamb-dip technique has been employed for recording the rotational spectra of three isotopic species of silyl fluoride, namely (28)SiH3F, (29)SiH3F, and (30)SiH3F, in order to improve the knowledge of their spectro- scopic parameters as well as to try to resolve their hyperfine structure. High-level quantum-chemical computations using state-of-the-art coupled-cluster techniques together with core-polarized correla- tion-consistent basis sets have been employed to provide reliable reference values for the hyperfine parameters involved and have been used to guide the experimental investigation. Analysis of the exper- imental spectra allowed to improve the accuracy of the known spectroscopic parameters as well as to determine for the first time sextic and octic centrifugal-distortion constants
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