Physical chemistry computational methods

Per-Olov Lowdin. Guest Editors N. Yngve Ohrn, John R. Sabin, and Michael C. Zerner, Quantum Chemistry, Solid-State Physics, and Computational Methods. Proceedings of the International Symposium held at St. Augustine, Florida, March 17-24, 1990, in Int. J. Quantum Chem., Quantum Chem. Symp., No. 24, Wiley, New York, 1990. 

The last chapter differs from the rest of the book. Here we present a collection of case histories , in which we discuss examples from the chemistry research literature on what has been learned about chemical structures using all appropriate physical and computational methods. It draws on what has been derived and explained in Chapters 2-11, but from the point of view of the chemist who has a compound and wants to know as much as possible about it rather than that of someone with a particular instrument or simulation software... 

A highly readable account of early efforts to apply the independent-particle approximation to problems of organic chemistry. Although more accurate computational methods have since been developed for treating all of the problems discussed in the text, its discussion of approximate Hartree-Fock (semiempirical) methods and their accuracy is still useful. Moreover, the view supplied about what was understood and what was not understood in physical organic chemistry three decades ago is... 

In the end, mass spectrometry and ion techniques will continue to be powerful tools for the investigation of the structure, bonding, energetics, and reactivity of unusual organic molecules. New sophisticated techniques will continue to be developed and applied to interesting problems in physical organic chemistry. These studies, along with the continued improvements in computational methods (Chapter 9), provide means to obtain very detailed and accurate descriptions of chemical reactions. 

The structure of the 2-norbornyl cation has been a focal point of controversy in physical organic chemistry. Experimental NMR spectroscopy and computational methods have been the decisive tools, favoring the hypercoordinated symmetric bridged structure 30, a protonated nortricyclane.79 The tricoordinated 2-norbornyl cation 31 is not a local minimum (MP2/6-31G(d)) on the energy surface.80... 

The previous result is an important one. It indicates that there can be yet another fruitful route to describe lipid bilayers. The idea is to consider the conformational properties of a probe molecule, and then replace all the other molecules by an external potential field (see Figure 11). This external potential may be called the mean-field or self-consistent potential, as it represents the mean behaviour of all molecules self-consistently. There are mean-field theories in many branches of science, for example (quantum) physics, physical chemistry, etc. Very often mean-field theories simplify the system to such an extent that structural as well as thermodynamic properties can be found analytically. This means that there is no need to use a computer. However, the lipid membrane problem is so complicated that the help of the computer is still needed. The method has been refined over the years to a detailed and complex framework, whose results correspond closely with those of MD simulations. The computer time needed for these calculations is however an order of 105 times less (this estimate is certainly too small when SCF calculations are compared with massive MD simulations in which up to 1000 lipids are considered). Indeed, the calculations can be done on a desktop PC with typical... 

Chapter 3, by Nicolaides and Tomioka, on the generation and characterization of biscarbenes, bisnitrenes, and carbenonitrenes illustrates how computational methods can serve as a valuable tool in understanding highly reactive intermediates. Given that many of the high-level computations can be performed on desktop computers, computation is likely to become a more common tool in physical organic chemistry laboratories. A future volume in this series will be devoted to computational methods in photochemistry. 

Progress in Theoretical Chemistry and Physics is made at different rates in these various research fields. The aim of this book series is to provide timely and in-depth coverage of selected topics and broad-ranging yet detailed analysis of contemporary theories and their applications. The series will be of primary interest to those whose research is directly concerned with the development and application of theoretical approaches in the chemical sciences. It will provide up-to-date reports on theoretical methods for the chemist, thermodynamician or spectroscopist, the atomic, molecular or cluster physicist, and the biochemist or molecular biologist who wish to employ techniques developed in theoretical, mathematical or computational chemistry in their research programmes. It is also intended to provide the graduate student with a readily accessible documentation on various branches of theoretical chemistry, physical chemistry and chemical physics. 

Computational chemistry offers many advantages to teachers of physical chemistry. It can help students learn the material and develop critical thinking skills. As noted before, most students will probably use some sort of computational method in their chemistry careers, so it provides students with important experience. Furthermore, computational chemistry is much more accessible to undergraduate students than it was a decade ago. Desktop computers now have sufficient resources to calculate the properties of illustrative and interesting chemical systems. Computational software is also becoming more affordable. Students can now use computers to help them visualize and understand many aspects of physical chemistry. However, physical chemistry is also an experimental science, and computational models are still judged against experimental results. 

Problem solving is an important and integral part of physical chemistry in addition to the concepts, principles and methods. There is a vast range of problems closed problems, with one answer open problems, which can have more than one answer and for which data may not be supplied problems that can be solved by pencil-and-paper or by the computer problems that need experiment in order to be solved and real-life problems versus scientific problems or even thought problems. A thorough classification of problem types has been made by Johnstone (107). 

One of the points made in Schwenz and Moore was that the physical chemistry laboratory should better reflect the range of activities found in current physical chemistry research. This is reflected in part by the inclusion of modem instrumentation and computational methods, as noted extensively above, but also by the choice of topics. A number of experiments developed since Schwenz and Moore reflect these current topics. Some are devoted to modem materials, an extremely active research area, that I have broadly construed to include semiconductors, nanoparticles, self-assembled monolayers and other supramolecular systems, liquid crystals, and polymers. Others are devoted to physical chemistry of biological systems. I should point out here, that with rare exceptions, I have not included experiments for the biophysical chemistry laboratory in this latter category, primarily because the topics of many of these experiments fall out of the range of a typical physical chemistry laboratory or lecture syllabus. Systems of environmental interest were well represented as well. 

A review of the Journal of Physical Chemistry A, volume 110, issues 6 and 7, reveals that computational chemistry plays a major or supporting role in the majority of papers. Computational tools include use of large Gaussian basis sets and density functional theory, molecular mechanics, and molecular dynamics. There were quantum chemistry studies of complex reaction schemes to create detailed reaction potential energy surfaces/maps, molecular mechanics and molecular dynamics studies of larger chemical systems, and conformational analysis studies. Spectroscopic methods included photoelectron spectroscopy, microwave spectroscopy circular dichroism, IR, UV-vis, EPR, ENDOR, and ENDOR induced EPR. The kinetics papers focused on elucidation of complex mechanisms and potential energy reaction coordinate surfaces. 

New directions in teaching physical chemistry should also expose students to the fundamentals of computational chemistry and the more modem methods used in the determination of rates and mechanisms of chemical reactions. 

Computational methods are of increasing importance in the chemical sciences. This paper describes a computational chemistry laboratory course that has been developed and implemented at the University of Michigan as part of the core physical chemistry curriculum. This laboratory course introduces students to the principle methods of computational chemistry and uses these methods to explore and visualize simple chemical problems. 

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