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\title{EFFECTIVE DIPOLE MOMENT MODEL FOR AXIALLY SYMMETRIC
$C_{3v}$ MOLECULES: APPLICATION TO THE PRECISE STUDY OF ABSOLUTE
LINE STRENGTHS OF THE $\nu_6$ FUNDAMENTAL OF CH$_3$Cl }

\author{\underline{\FirstName O. N. \LastName ULENIKOV}, \underline{\FirstName E. S. \LastName
BEKHTEREVA}, \underline{\FirstName O. V. \LastName GROMOVA},
\underline{\FirstName A. L. \LastName FOMCHENKO}}
\address{National Research Tomsk Polytechnic University, 30, av. Lenina, 634050 Tomsk,
Russia}

\author{\underline{\FirstName C. \LastName SYDOW} and
  \underline{\FirstName S. \LastName BAUERECKER}}
\address{Institut f\"{u}r Physikalische und Theoretische Chemie, Technische Universit\"{a}t Braunschweig, D - 38106,
Braunschweig, Germany}



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\pagestyle{fancy} \fancyhead[C]{{\footnotesize \textit{HRMS, 28th
Colloquium, Dijon, France,
  Aug. 28 - Sept. 1, 2023}}}
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\index{CH$_3$Cl} \index{absolute line strengths}

The effective dipole moment model for molecules of axial C$_{3v}$
symmetry is derived on the basis of the symmetry properties of a
molecule which, on the one hand, is of the same order of efficiency
(but much simpler and clearer in applications) as the analogous
models derived on the basis of the irreducible tensorial sets theory
and, on the other hand, mathematically more correct in comparison
with concepts like the Herman--Walles function used in the models.
As an application of the general results obtained, we discuss high
resolution infrared spectra of CH$_3$Cl, recorded with the
Z\"{u}rich prototype ZP2001 (Bruker IFS125 HR) Fourier transform
infrared spectrometer at a resolution of 0.001 cm$^{-1}$ and
analyzed in the region of 880--1190 cm$^{-1}$. Absolute strengths of
more than 2800 transitions (2081 lines) were obtained from the fit
of their shapes both with Voigt and Hartmann--Tran profiles, and
parameters of the effective dipole moment of the $\nu_6$ band were
determined by the computer code SYMTOMLIST (SYMmetric TOp Molecules:
LIne STrengths), created on the basis of a derived theoretical
model. As the first step of the analysis of the experimental data,
assignments of the recorded lines were made. A total of about 5180
transitions with $J^{\textrm{max}}$ = 68, $K^{\textrm{max}}$ = 21
were assigned to the $\nu_6$ band. The weighted fit of 2077 upper
energy values obtained from the experimentally recorded transitions
was made with a Hamiltonian which takes into account different types
of ro--vibrational effects in doubly degenerate  vibrational states
of the $C_{3v}-$symmetric molecule. As the result, a set of 25
fitted parameters was obtained which reproduces the initial 2077
upper ''experimental`` ro--vibrational energy values with a root
mean square deviation $d_{\textrm{rms}} = 4.7 \times 10^{-5}$
cm$^{-1}$. At the second step of the analysis, the computer code
SYMTOMLIST was used for determination of the parameters of the
derived effective dipole moment model. Six effective dipole moment
parameters were obtained from the weighted fit procedure which
reproduces absolute experimental strengths of the 2804 initial
experimental transitions with a relative $d_{\textrm{rms}} = 3.4
\%$.

The study was financially supported by the TPU development program
''Priority 2030`` (project NIP/EB--010--375--2023).


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