LITERATURE SEARCH ASSIGNMENT

A) Properties of diatomic molecules

A diatomic molecule is a molecule composed of two atoms. For homonuclear diatomics the atoms are both from the same element, while for heteronuclear diatomics the atoms are from two different elements.

When the two atoms forming the covalent bond are far apart they do not interact. We take that as the zero of the potential. As the atoms approach they can share one or more pairs of electrons, forming a covalent bond and lowering the energy. If the atoms approach too close to one another, nuclear-nuclear repulsion causes the energy to become large.

re - equilibrium bond distance (minimum in potential)

De - dissociation energy (difference in energy between minimum and value at r = .

Classically systems such as diatomic molecules can have any value for energy within a particular range of values. However, one consequence of quantum mechanics is that atomic and molecular systems often can only have certain specific values for energy. These energy levels are often labeled by quantum numbers that identify the states of the system.

For example, for the electron in a hydrogen atom, the allowed values for energy (energy levels) are given by the Rydberg formula:

En = - (RH)/n2 RH = 109677. cm-1

n = 1, 2, 3, ...

e - vibrational constant. Related to the curvature of the poten-tial at r = re.

Ev = (v + 1/2) e v = 0, 1, 2, … harmonic oscillator approximation

Ev = (v + 1/2) e - (v + 1/2)2 exe + (v + 1/2)3 eye + ... v = 0, 1, 2, …vmax

The potential energy curve for a diatomic molecule can be characterized by the values of parameters such as e, re, De, and so forth. These may be found either from experiment or theory.

Experimental - A spectroscopic technique, such as microwave, infrared, Raman, or UV-visible spectroscopy is used to find the values for the energy levels of the molecule, from which the potential energy curve for the molecule can be obtained.

Theoretical - Molecular orbital calculations using approximation methods for solving the Schrodinger equation are carried out to find the potential energy curve.

B) Literature search report

In this assignment you are to find two experimental and two theoretical values for e and re.

1) You must use primary sources for your values.

Primary source - Values reported by people who did the experimental work or calculations.

Secondary source - Values reported are quoted from another source.

CAUTION - The NIST values are taken from Huber and Herzberg and is therefore a secondary source. You may go to the references that appear in NIST, which are usually primary sources (though the most recent references are ~ 1975).

2) For the theoretical values you must report the method used to do the calculation. If several methods are used, only report two of the methods (and this will only count as one source).

3) Diatomic molecules have different ways in which their electrons can be arranged, and so different electronic states with different values for e and re. You are interested in the ground (lowest energy) state. This will usually be labeled as the X state, with higher energy states labeled as A, B, C, …or a, b, c, … If there may be several isotopic forms of the diatomic, report values for the most common isotopes.

4) You will be graded in part on how recent your references are. Therefore you should focus on the most recent literature references.

5) You must use the format given in the sample handout to report your information. Note the following:

a) Values for re should be reported in nm, and values for e should be reported in cm-1. They should be reported to the same number of significant figures as in the original source, and, uncertainties, if reported, should also be included.

b) Experimental values should reported first.

c) The name(s) and the year in which the source was published should be listed in the body of the report, in order, from most recent to least recent.

d) There should be a list of references at the end of the report. Sources should be listed here alphabetically by first author. A complete reference must be given: author(s), year published, title of paper, journal name, volume number, page numbers.

C) Searching the scientific literature

We will focus on three places to search the scientific literature which are appropriate for this assignment. There are a number of other ways of searching the literature that can also be used.

NIST (National Institute of Standards and Technology) is a US Government agency that, among other things, compiles information concerning science and technology.

Here we would type in the formula of our diatomic molecule and check the box “constants of diatomic molecules”

The most common isotopic combination is 1H32S.

SCI (Science Citation Index) allows you to find papers that have referenced other papers. This is a good (though tedious) way of tracking down recent work.

In the above example we are using the 1970 paper by Acquista and Schoen as our starting point, and finding more recent papers that have used this paper as a reference.

There are 19 papers that have used the Acquista paper as a reference. We can now find the authors and titles for those papers.

Reference 14 appears interesting but might be difficult to find. Reference 15 also appears interesting (and in fact contains information on e and re).

SCI also allows topic (keyword) and author searches.

SciFinder Scholar is the ACS search system (largely replacing Chemical Abstracts). It is available on the computer in CP 375, most faculty computers, and some of the computers on the second floor of the Green Library.

The most useful searching method for this assignment is by mole-cular formula.

Various combinations of charge and isotopic composition are listed separately.

We can limit our searching to “spectral properties” though this sometimes may exclude useful references.

The papers by Ram et al., and by Morino et al. both appear interesting. In SciFinder Scholar you can read the abstracts of the papers and decide if they are worth looking into.

The paper by Ram et al. reports experimental values for e and re. The paper itself might be useful to read, as it might contain additional references that you could use. You could also now use the Ram paper in an SCI search.

Most of the more common chemistry journals are available both in hard copy (in the Green Library) and online (via the FIU Library homepage). In some cases the online resources are limited to article abstracts, or are not complete.