Studying the Oscillation of Hydrochloric Acid via Gas Phase Vibrational-Rotational Infrared Spectroscopy Josiah Matthew Bob Jones University Chemistry Department 1700 Wade Hampton BLVD Greenville, SC 29614 Abstract The following experiment studies the oscillations of hydrochloric acid using vibrational rotational infrared spectroscopy. The spectral data is fitted to a polynomial from which the moment of inertia and inter-nuclear distance are both calculated. These data can be used to analyze how well an anharmonic oscillator fits our data. Most of the data corresponds well to literature data, except for values predicted for an anharmonic oscillator. This suggest that HCl best fits the model of a harmonic oscillator rigid rotor, and demonstrates very little anharmonicity. Introduction The purpose of this experiment is to study the harmonic and anharmonic oscillations of HCl. A quantum harmonic oscillator is a quantum mechanical approximation of a classical harmonic oscillator which follows Hookβs law. In this model, a chemical bond is treated like a spring, where a restoring force acts upon a molecule when it is displaced from equilibrium. A harmonic oscillating molecule is quantized and only has specifically allowed energy levels. The simplest model of a harmonic oscillator is the rigid rotor, which assumes that atoms are joined by a rigid weightless rod which does not distort under rotational stress. A chemical bond is not truly rigid and therefore does stretch some when rotated. Anharmonicity is a deviation from harmonicity where the actual potential is different from the harmonic potential and the actual vibrational energy levels are not as predicted by harmonic oscillation due to centrifugal distortion. This experiment uses vibrational-rotational infrared spectroscopy to determine study the harmonic-oscillator rigid rotor model as it applies to HCl gas. Infrared rotational-vibrational (rovibrational) spectroscopy is used extensively in physical chemistry. The rovibrational spectra of polycyclic aromatic hydrocarbons (PAHs) have been studied and PAHs are thought to be responsible for some of the unidentified spectral bands in interstellar emissions 1 . One study that is similar to this current experiment uses rovibrational spectroscopy on various isotopologues of carbon monoxide in order to test molecular rotor theories by calculating rotational constants and predicting the center of mass and center of
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Studying the Oscillation of Hydrochloric Acid via Gas Phase Vibrational-Rotational Infrared
Spectroscopy
Josiah Matthew
Bob Jones University Chemistry Department
1700 Wade Hampton BLVD
Greenville, SC 29614
Abstract
The following experiment studies the oscillations of hydrochloric acid using vibrational
rotational infrared spectroscopy. The spectral data is fitted to a polynomial from which the
moment of inertia and inter-nuclear distance are both calculated. These data can be used to
analyze how well an anharmonic oscillator fits our data. Most of the data corresponds well to
literature data, except for values predicted for an anharmonic oscillator. This suggest that HCl
best fits the model of a harmonic oscillator rigid rotor, and demonstrates very little
anharmonicity.
Introduction
The purpose of this experiment is to
study the harmonic and anharmonic
oscillations of HCl. A quantum harmonic
oscillator is a quantum mechanical
approximation of a classical harmonic
oscillator which follows Hookβs law. In this
model, a chemical bond is treated like a
spring, where a restoring force acts upon a
molecule when it is displaced from
equilibrium. A harmonic oscillating
molecule is quantized and only has
specifically allowed energy levels. The
simplest model of a harmonic oscillator is
the rigid rotor, which assumes that atoms
are joined by a rigid weightless rod which
does not distort under rotational stress. A
chemical bond is not truly rigid and
therefore does stretch some when rotated.
Anharmonicity is a deviation from
harmonicity where the actual potential is
different from the harmonic potential and
the actual vibrational energy levels are not
as predicted by harmonic oscillation due to
centrifugal distortion. This experiment uses
vibrational-rotational infrared spectroscopy
to determine study the harmonic-oscillator
rigid rotor model as it applies to HCl gas.
Infrared rotational-vibrational
(rovibrational) spectroscopy is used
extensively in physical chemistry. The
rovibrational spectra of polycyclic aromatic
hydrocarbons (PAHs) have been studied
and PAHs are thought to be responsible for
some of the unidentified spectral bands in
interstellar emissions1. One study that is
similar to this current experiment uses
rovibrational spectroscopy on various
isotopologues of carbon monoxide in order
to test molecular rotor theories by
calculating rotational constants and
predicting the center of mass and center of
interaction of the molecule2. Rovibrational
spectroscopy has been important in
determining the structural properties,
reaction kinetics, and dynamics of radicals
such as the phenyl radical, which has a
variety of uses due to its high reactivity, and
is considered the most important aromatic
radical in all of chemistry3. In addition,
rovibrational spectroscopy has been used in
determining the presence of
dichlorodifluoromethane (CFC-12), a
significant greenhouse gas, in the
atmosphere4. The chemical and physical
properties of SiO have also been discovered
using rovibrational spectroscopy, and this
data is important to its role in interstellar
space5.
This experiment uses a gas phase
infrared rovibrational spectrum to
determine the harmonic properties of HCl
gas. Because chlorine exists in isotopes 35Cl
(~75 %) and 37Cl (~25 %), two resolved
features are observed for each absorption
peak. The spectrum shows absorbances for
vibrational transitions following the
selection rule for a change in value of the
rotational quantum number of βπ½ = Β±1.
The spectrum is divided into R and P
branches, where βπ½ = 1 for the R branch,
and βπ½ = β1 for the P branch. This
experiment is concerned with transitions
from the ground state Jββ values to the first
excited state Jβ. The quantity m is defined as
π = π½β²β² + 1 for the R branch, and π = βπ½β²β²