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\title{ISOTOPIC SUBSTITUTION IN POLYATOMIC MOLECULES:
COMPARATIVE LINE POSITION AND LINE STRENGTH ANALYSIS OF THE
$\nu_2/\nu_4$ DYAD OF ${}^{12}$CD$_4$ AND ${}^{13}$CD$_4$ }

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

\author{\underline{\FirstName C. \LastName SYDOW}, \underline{\FirstName C. \LastName MAUL} 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{CD$_4$} \index{line positions, line strengths}

A highly accurate ro--vibrational analysis of FTIR spectra (line
positions of ${}^{13}$CD$_4$ and line strengths of both the
${}^{12}$CD$_4$ and ${}^{13}$CD$_4$ species) is presented. The
high--resolution infrared spectra of both molecules were measured
with a Bruker IFS125 HR Fourier transform infrared spectrometer at
an optical resolution of 0.003 cm$^{-1}$ and analyzed in the regions
of 800--1400 cm$^{-1}$ where the $\nu_2/\nu_4$ dyad is located. The
number of 901 transitions with $J^{\textrm{max}} = 23$ were assigned
to the $\nu_4$ and $\nu_2$ bands of ${}^{13}$CD$_4$. The weighted
fit of experimental line positions was made using the Hamiltonian
model which takes into account the resonance interactions between
the $(0001, F_2)$ and $(0100, E)$ vibrational states. As a result,
set of 18 fitted parameters of the $(0001, F_2)$/$(0100, E)$
vibrational states of ${}^{13}$CD$_4$ was determined which reproduce
the initial 901 experimental ro--vibrational line positions with the
$d_{\textrm{rms}} = 2.59 \times 10^{-4}$ cm$^{-1}$, which is close
to the experimental uncertainty of the recorded spectra. The
analysis of 1557 experimental lines of the dyad of ${}^{12}$CD$_4$
and 131 lines of the dyad of ${}^{13}$CD$_4$ was fulfilled with the
Hartmann--Tran profile to simulate the measured line shape and to
determine experimental line intensities. Sets of 6/1 varied
effective dipole moment parameters of
${}^{12}$CD$_4$/${}^{13}$CD$_4$ are determined which reproduce the
initial 1557/131 line strengths with the $d_{\textrm{rms}} = 4.80
\%$ and $4.21 \%$.\\

The study was financially supported by Volkswagen Foundation.

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