Laboratory Report: Determination of Fundamental Vibration Frequency and Isotope Effects in HCl and DCl Molecules

Categories: Physics

Vibration spectroscopy is a powerful technique for elucidating molecular structure and understanding isotope effects. This laboratory aims to determine the fundamental vibration frequency and bond length for H35Cl, H37Cl, D35Cl, and D37Cl, and to compare the isotope effects with theoretically predicted values. Vibration spectroscopy is crucial in environmental chemistry, as it helps study the absorption of infrared light by greenhouse gases like CO2, H2O, and CH4. This experiment focuses on normal modes that exhibit a changing dipole moment, making them observable through infrared absorption, i.

e., IR active.


  1. Fundamental Vibration Frequency:
  1. Fundamental Vibration Frequency: The fundamental vibration frequency (ν~0​) is related to the force constant (k) and reduced mass (μ) of the vibrating system by the equation: ν~0​=2π1​(μk​)0.5
  2. Isotope Effects: The fundamental vibration frequency changes with isotopic substitution due to altered reduced masses. For HCl and DCl, the isotopic substitution is in the hydrogen atom. The shift in frequency (Δν) can be calculated using the reduced masses of the isotopologues: Δν=ν~0​(H35Cl or D35Cl)−ν~0​(H37Cl or D37Cl)
  1. Rotational Constant: The rotational constant (B) is linked to the moment of inertia (I) and the bond length (r) by the formula: B=8π2cIh​
  2. Bond Length: The bond length can be determined from the rotational constant using: r=(8π2cBh​)0.

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Experimental Procedure:

  1. Sample Preparation: Prepare samples of H35Cl, H37Cl, D35Cl, and D37Cl in suitable containers.
  2. Infrared Absorption Spectroscopy: Record the infrared absorption spectra for HCl and DCl using appropriate equipment.
  3. Analysis: Identify the peaks corresponding to H35Cl, H37Cl, D35Cl, and D37Cl.

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    Measure the fundamental vibration frequencies for each isotope.

  4. Calculation of Isotope Effects: Use the measured frequencies to calculate the isotope effects according to the formula mentioned earlier.
  5. Prediction of Rotational Constants: Predict the rotational constants for each isotopologue using theoretical values and compare them with the experimental results.
  6. Bond Length Determination: Calculate bond lengths for HCl and DCl using the experimentally determined rotational constants.

Compare the experimental values with the theoretically predicted values. Discuss any discrepancies and potential sources of error. Consider the implications of the isotope effects on molecular properties.

Conclusion: Summarize the findings and highlight the significance of the experiment in understanding isotope effects on molecular vibrations. Discuss potential applications and future research directions.

References: List any references or theoretical frameworks used in the calculations and predictions. Ensure proper citation of sources.

This laboratory report provides a comprehensive overview of the experimental procedure, theoretical background, and results obtained in the determination of fundamental vibration frequencies and isotope effects in HCl and DCl molecules.

In the realm of diatomic molecules, their vibrational energy within the harmonic oscillator approximation is defined by the expression:


Here, ν0​ signifies the fundamental vibration frequency in s−1−1, and ν represents the vibration quantum number. The rotational energy, in the rigid-rotor approximation, is given by:


Where B0​ is the rotational constant in cm−1−1, J is the rotation quantum number, and ℏℏ is the reduced Planck constant. Each rotational level has a degeneracy of gJ​=2J+1. The crucial connection with molecular structure involves ν0​ through the relationship with the bond force constant k:


Here, μ is the reduced mass given by μ=m1​+m2​m1​m2​​. The rotational constant is also related to the bond length (R0​) through:​

Isotope effects, especially upon deuteration, are crucial for understanding molecular vibrations. The fundamental vibration frequency ratio for two isotopically substituted molecules is given by:


For example, in transitioning from 35Cl to 35Cl, the reduced mass increases by a factor of about 2, leading to a decrease in the fundamental vibration frequency by a similar factor. The ratio of rotational constants for isotopically substituted molecules with the same bond length is given by:


However, these theoretical ratios do not account for anharmonicity or centrifugal distortion, introducing small deviations between theoretical and experimental values.

In summary, these theoretical frameworks provide a foundation for understanding the fundamental vibration frequencies, rotational constants, and isotope effects in diatomic molecules, forming the basis for the interpretation of experimental data in the laboratory setting.

Updated: Feb 29, 2024
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Laboratory Report: Determination of Fundamental Vibration Frequency and Isotope Effects in HCl and DCl Molecules. (2024, Feb 29). Retrieved from

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