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An IR Investigation of the CO Dipole Direction and Other Properties

Posted on May 11 2006 by Kieran F. Lim

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Experiment Overview

Spectroscopy is a direct probe of molecular properties. The novelty of this experiment is the placement of a nearstandard Fourier-transform infrared (FT-IR) spectroscopy in an applied context of relevenace to physical chemistry, toxicology and organometallic chemistry.

For many years, there was disagreement about the direction of the carbon monoxide (CO) molecular dipole, which had major implications for how CO acts as a poisonous gas (in its binding to haemoglobin), or as a ligand in organometallic complexes.

Neither Lewis structure is very satisfactory. Structure I is a zwitterion, which obeys the "octet rule", but places the negative charge on the more electropositive atom, and vice versa. Structure II minimises the formal charge, but does not obey the "octet rule". Structures I and II predict opposite directions for molecular dipole.

Structures I and II also differ in their bond order. In this laboratory exercise, infrared (IR) spectroscopy is used to determine the bond order of CO, and hence infer the direction of the molecular dipole, as follows:

The vibrational transitions are used to estimate the harmonic frequency. The bond order can be determined from the frequency-bond-order relationship.

The vibrational transitions are used to determine the anharmonicity and harmonic frequency. The bond dissociation energy can be determined from the harmonic frequency and the anharmonicity. The bond order can be determined from the bond-strength-bond-order.

The rotational fine structure of the vibrational transitions is used to determine the rotational constant, and hence the molecular bond length. The bond order can be determined from the bond-length- bond-order relationship. This laboratory exercise enables three related but independent measures of the bond order of CO and hence infer the direction of the molecular dipole from the Lewis
tructures I and II.

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Recent Discussion:

"This is a fantastic experiment...."

Posted: Jan 23, 2011 by Jason B

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