Molecular Magnetochemistry

Sergey Vulfson PhD

• Publisher: CRC Press
• Year: 2019
• eBook ISBN: 9781000722819
• Topic: Physical Sciences
• Subtopic: Chemistry
• Index: Chemistry

Investigating the relationship between the magnetic properties and structure of molecules, molecular magnetochemistry, is an area of growing interest to scientists in a variety of fields, including physical, organic and inorganic chemistry, molecular physics, and biophysics.
For the first time, systematic results on magnetic properties of molecules such as mean magnetic susceptibility, their anisotropies and principal magnetic axes are presented. Molecular Magnetochemistry is a comprehensive and up-to-date view on experimental methods not covered in previous volumes, including the Zeeman effect in vapor phase and magnetic birefringence of diamagnetic systems (Cotton-Mouton Effect). The relationship between magnetic and related electrical phenomena is also described, summing up experimental data on magnetic and electrical anisotropies and components of molecular quadrupole moments.

CHAPTER 1

DETERMINATION OF AVERAGE MAGNETIC SUSCEPTIBILITIES

The experimental methods for determination of the average, i.e. scalar, magnetic susceptibilities can be divided into three groups.

In the first group there are methods based on measuring the force with which the sample interacts with a magnetic field. For example, the magnetic susceptibility of a substance placed in a homogeneous magnetic field, can be found with the help of the Gouy balance,^{1,2,3,4 and 5} the capillary Quincke method,^2,4 or the viscosimeter method.^6,7 The Faraday method and its modifications, such as Fereday-Domenicali^5,8 or Gordon methods,⁹ are based on measuring the magnetic susceptibility in non-homogeneous field

The second group is composed of induction methods.¹ These are based on measurements of the electromotive force produced by vibrating sample or of the inductance of a coil after the sample is inserted into it

Methods based on the analysis of ¹H chemical shifts in NMR form the third group.^{10,11,12,13,14 and 15} The original method of the determination of magnetic susceptibilities of paramagnetic solid compounds by high field Mossbauer spectroscopy is also included into this group.¹⁶

The author does not consider the specifics of each group. The details of each method can be found in the literature (for example^{1,2,3,4 and 5}). Instead, we will focus only on principles of the most widespread and the most promising experimental approaches with the identification of their special features, advantages, and disadvantages.

1.1. Force Methods

1.1.1. The Gouy Method

The Gouy method is the simplest to implement and, therefore, the most widely used. Gouy set up includes an electromagnet which generates a homogeneous field of 5-25 kG (depending on the magnetic properties of samples); an analytical balance, with the precision of 10⁻⁵ g; and a cylindrical (usually made from glass) tube of constant diameter which is suspended from the analytical balance by a quartz rod and is positioned between the poles of the magnet. There are two types of tubes.

The first is slightly longer in length than a diameter of interpolar pieces used. This tube is filled almost completely with the substance under study, and is positioned so that its end is in the center of interpolar space (Figure 1.1 a).

The second, called “double differential tube”, contains a partition in the middle, with the substance (usually its solution) placed into one half, and with the standard substance or solvent placed into another half. The ends of this tube are placed symmetrically beyond poles, so that the partition is in the center of the magnet (Figure 1.1 b).

The force acting on a sample is written as

$\text{F}=\left({\chi }_{\text{v}}-{\chi }_{\text{v}}^0\right)\text{S}\left({\text{H}}^2-{\text{H}}_0^2\right)/2=\text{gΔM,}$

(1.1)

where

$\text{ΔM=}\left({\text{M}}_{\text{0}}-{\text{M}}_{\text{H}}\right)$

Here χ_v is the volume magnetic susceptibility of the sample, H is the maximum magnetic field strength, H₀ is the field strength near of the remote end of the tube (because H ≫ H₀, the contribution of H₀ can be neglected);${\chi }_{\text{v}}^{\text{0}}$ is the volume magnetic susceptibility of a medium (air) displaced by the sample; S is the cross-sectional area of the sample; M₀ and M_H are the masses of the sample in the absence and in the presence of the field, respectively, and g is the acceleration of gravity.

The Gouy method permits the measurement of both the “absolute” and the “relative” values of χ. In the “relative” experiment, the standard substance is placed in the same tube and the same field as the sample. These conditions eliminate the necessity to measure the values of H and S. As a result, the specific susceptibility can be expressed⁵ as

$\chi =\left({\chi }^0{\text{d}}^0/\text{d}\right)+\left({\chi }^{\text{st}}-{\chi }^0{\text{d}}^0/{\text{d}}^{\text{st}}\right){\text{Md}}^{\text{st}}/\left({\text{M}}^{\text{st}}\text{d}\right).$

(1.2)

Here d, d^st, and d⁰ are densities of the sample, the standard substance, and the medium (air); M and M^st are the masses of the sample and the standard substance, respectively. Co...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Dedication
• Table of Contents
• Preface
• Illustration Acknowledgments
• Introduction Magnetic Susceptibility as a Molecular Tensor Property
• 1 Determination of Average Magnetic Susceptibilities
• 2 Methods of Determination of Anisotropic Magnetic Susceptibilities of Molecules
• 3 Relation Between Magnetic and Electric Properties of Molecules
• 4 Tensor Additivity of Molecular Electron Properties
• 5 Magnetic Susceptibilities and Interatomic Interaction Effects
• 6 Determination of Three-Dimensional Structure of Molecules and Complexes
• References
• Subject Index