Joel M. Bowman | |
|---|---|
| Born | Jan. 16, 1948 |
| Education | University of California, Berkeley California Institute of Technology |
| Scientific career | |
| Institutions | Emory University |
| Doctoral advisor | Aron Kuppermann |
Joel Mark Bowman is an American physical chemist and educator. He is the Samuel Candler Dobbs Professor of Theoretical Chemistry at Emory University. [1]
Bowman is the author or co-author of more than 600 publications and is a member of the International Academy of Quantum Molecular Sciences. He received the Herschbach Medal [2] . He is a fellow of the American Physical Society [3] and of the American Association for the Advancement of Science. [1]
His research interests are in basic theories of chemical reactivity. [1] His AAAS fellow citation cited him “for distinguished contributions to reduced dimensionality quantum approaches to reaction rates and to the formulation and application of self-consistent field approaches to molecular vibrations.” [1]
Bowman is well known for his contributions in simulating potential energy surfaces for polyatomic molecules and clusters. Approximately fifty potential energy surfaces for molecules and clusters have been simulated employing his permutationally invariant polynomial method. [4]
Simulating potential energy surfaces (PESs) for reactive and non-reactive systems is of broad utility in theoretical and computational chemistry. Development of global PESs, or surfaces spanning a broad range of nuclear coordinates, is particularly necessary for certain applications, including molecular dynamics and Monte Carlo simulations and quantum reactive scattering calculations.
Rather than utilizing all of the internuclear distances, theoretical chemists often analytical equations for PESs by using a set of internal coordinates. For systems containing more than four atoms, the count of internuclear distances deviates from the equation 3N−6 (which represents the degrees of freedom in a three-dimensional space for a nonlinear molecule with N atoms). [5] [6] As an example, Collins and his team developed a method employing different sets of 3N−6 internal coordinates, which they applied to analyze the H+ CH4 reaction. They addressed permutational symmetry by replicating data for permutations of the H atoms. [7] In contrast to this approach, the PIP method uses the linear least-square method to accurately match tens of thousands of electronic energies for both reactive and non-reactive systems mathematically.
Generally, the functions used in fitting potential energy surfaces to experimental and/or electronic structure theory data are based on the choice of coordinates. Most of the chosen coordinates are bond stretches, valence and dihedral angles, or other curvilinear coordinates such as the Jacobi coordinates or polyspherical coordinates.[ citation needed ] There are advantages to each of these choices. [4] In the PIP approach, the N(N − 1)/2 internuclear distances are utilized. This number of variables is equal to 3N −6 (or 3N − 5 = 1 for diatomic molecules) for N = 3, 4 and differs for N ≥ 5. Thus, N = 5 is an important boundary that affects the choice of coordinates. An advantage of employing this variable set is its inherent closure under all permutations of atoms. This implies that regardless of the order in which atoms are permuted, the resulting set of variables remains unchanged. However, the main focus pertains to permutations involving identical atoms, as the PES must be invariant under such transformations. [4]
PIP utilizing Morse variables of the form , where is the distance between atoms and and is a range parameter) offers a method for mathematically characterizing high-dimensional PESs. By fixing the range parameter in the Morse variable, the PES can be determined through linear least-squares fitting of computed electronic energies for the system at various structural arrangements. The adoption of a permutationally invariant fitting basis, whether in the form of all internuclear distances or transformed variables like Morse variables, facilitates the attainment of accurate fits for molecules and clusters. [8]
Diatomic molecules are molecules composed of only two atoms, of the same or different chemical elements. If a diatomic molecule consists of two atoms of the same element, such as hydrogen or oxygen, then it is said to be homonuclear. Otherwise, if a diatomic molecule consists of two different atoms, such as carbon monoxide or nitric oxide, the molecule is said to be heteronuclear. The bond in a homonuclear diatomic molecule is non-polar.
Infrared spectroscopy is the measurement of the interaction of infrared radiation with matter by absorption, emission, or reflection. It is used to study and identify chemical substances or functional groups in solid, liquid, or gaseous forms. It can be used to characterize new materials or identify and verify known and unknown samples. The method or technique of infrared spectroscopy is conducted with an instrument called an infrared spectrometer which produces an infrared spectrum. An IR spectrum can be visualized in a graph of infrared light absorbance on the vertical axis vs. frequency, wavenumber or wavelength on the horizontal axis. Typical units of wavenumber used in IR spectra are reciprocal centimeters, with the symbol cm−1. Units of IR wavelength are commonly given in micrometers, symbol μm, which are related to the wavenumber in a reciprocal way. A common laboratory instrument that uses this technique is a Fourier transform infrared (FTIR) spectrometer. Two-dimensional IR is also possible as discussed below.
In quantum chemistry and molecular physics, the Born–Oppenheimer (BO) approximation is the best-known mathematical approximation in molecular dynamics. Specifically, it is the assumption that the wave functions of atomic nuclei and electrons in a molecule can be treated separately, based on the fact that the nuclei are much heavier than the electrons. Due to the larger relative mass of a nucleus compared to an electron, the coordinates of the nuclei in a system are approximated as fixed, while the coordinates of the electrons are dynamic. The approach is named after Max Born and his 23-year-old graduate student J. Robert Oppenheimer, the latter of whom proposed it in 1927 during a period of intense ferment in the development of quantum mechanics.
Molecular dynamics (MD) is a computer simulation method for analyzing the physical movements of atoms and molecules. The atoms and molecules are allowed to interact for a fixed period of time, giving a view of the dynamic "evolution" of the system. In the most common version, the trajectories of atoms and molecules are determined by numerically solving Newton's equations of motion for a system of interacting particles, where forces between the particles and their potential energies are often calculated using interatomic potentials or molecular mechanical force fields. The method is applied mostly in chemical physics, materials science, and biophysics.
Molecular physics is the study of the physical properties of molecules and molecular dynamics. The field overlaps significantly with physical chemistry, chemical physics, and quantum chemistry. It is often considered as a sub-field of atomic, molecular, and optical physics. Research groups studying molecular physics are typically designated as one of these other fields. Molecular physics addresses phenomena due to both molecular structure and individual atomic processes within molecules. Like atomic physics, it relies on a combination of classical and quantum mechanics to describe interactions between electromagnetic radiation and matter. Experiments in the field often rely heavily on techniques borrowed from atomic physics, such as spectroscopy and scattering.
The Franck–Condon principle is a rule in spectroscopy and quantum chemistry that explains the intensity of vibronic transitions. The principle states that during an electronic transition, a change from one vibrational energy level to another will be more likely to happen if the two vibrational wave functions overlap more significantly.
In quantum chemistry, a conical intersection of two or more potential energy surfaces is the set of molecular geometry points where the potential energy surfaces are degenerate (intersect) and the non-adiabatic couplings between these states are non-vanishing. In the vicinity of conical intersections, the Born–Oppenheimer approximation breaks down and the coupling between electronic and nuclear motion becomes important, allowing non-adiabatic processes to take place. The location and characterization of conical intersections are therefore essential to the understanding of a wide range of important phenomena governed by non-adiabatic events, such as photoisomerization, photosynthesis, vision and the photostability of DNA. The conical intersection involving the ground electronic state potential energy surface of the C6H3F3+ molecular ion is discussed in connection with the Jahn–Teller effect in Section 13.4.2 on pages 380-388 of the textbook by Bunker and Jensen.
In the context of chemistry, molecular physics, physical chemistry, and molecular modelling, a force field is a computational model that is used to describe the forces between atoms within molecules or between molecules as well as in crystals. Force fields are a variety of interatomic potentials. More precisely, the force field refers to the functional form and parameter sets used to calculate the potential energy of a system on the atomistic level. Force fields are usually used in molecular dynamics or Monte Carlo simulations. The parameters for a chosen energy function may be derived from classical laboratory experiment data, calculations in quantum mechanics, or both. Force fields utilize the same concept as force fields in classical physics, with the main difference being that the force field parameters in chemistry describe the energy landscape on the atomistic level. From a force field, the acting forces on every particle are derived as a gradient of the potential energy with respect to the particle coordinates.
Car–Parrinello molecular dynamics or CPMD refers to either a method used in molecular dynamics or the computational chemistry software package used to implement this method.
In atomic, molecular, and optical physics and quantum chemistry, the molecular Hamiltonian is the Hamiltonian operator representing the energy of the electrons and nuclei in a molecule. This operator and the associated Schrödinger equation play a central role in computational chemistry and physics for computing properties of molecules and aggregates of molecules, such as thermal conductivity, specific heat, electrical conductivity, optical, and magnetic properties, and reactivity.
A molecular vibration is a periodic motion of the atoms of a molecule relative to each other, such that the center of mass of the molecule remains unchanged. The typical vibrational frequencies range from less than 1013 Hz to approximately 1014 Hz, corresponding to wavenumbers of approximately 300 to 3000 cm−1 and wavelengths of approximately 30 to 3 μm.
In chemistry, molecular symmetry describes the symmetry present in molecules and the classification of these molecules according to their symmetry. Molecular symmetry is a fundamental concept in chemistry, as it can be used to predict or explain many of a molecule's chemical properties, such as whether or not it has a dipole moment, as well as its allowed spectroscopic transitions. To do this it is necessary to use group theory. This involves classifying the states of the molecule using the irreducible representations from the character table of the symmetry group of the molecule. Symmetry is useful in the study of molecular orbitals, with applications to the Hückel method, to ligand field theory, and to the Woodward-Hoffmann rules. Many university level textbooks on physical chemistry, quantum chemistry, spectroscopy and inorganic chemistry discuss symmetry. Another framework on a larger scale is the use of crystal systems to describe crystallographic symmetry in bulk materials.
In the field of computational chemistry, energy minimization is the process of finding an arrangement in space of a collection of atoms where, according to some computational model of chemical bonding, the net inter-atomic force on each atom is acceptably close to zero and the position on the potential energy surface (PES) is a stationary point. The collection of atoms might be a single molecule, an ion, a condensed phase, a transition state or even a collection of any of these. The computational model of chemical bonding might, for example, be quantum mechanics.
In theoretical chemistry, an energy profile is a theoretical representation of a chemical reaction or process as a single energetic pathway as the reactants are transformed into products. This pathway runs along the reaction coordinate, which is a parametric curve that follows the pathway of the reaction and indicates its progress; thus, energy profiles are also called reaction coordinate diagrams. They are derived from the corresponding potential energy surface (PES), which is used in computational chemistry to model chemical reactions by relating the energy of a molecule(s) to its structure.
Giacinto Scoles is a European and North American chemist and physicist who is best known for his pioneering development of molecular beam methods for the study of weak van der Waals forces between atoms, molecules, and surfaces. He developed the cryogenic bolometer as a universal detector of atomic and molecule beams that not only can detect a small flux of molecules, but also responds to the internal energy of the molecules. This is the basis for the optothermal spectroscopy technique which Scoles and others have used to obtain very high signal-to noise and high resolution ro-vibrational spectra.
The dihydrogen cation or hydrogen molecular ion is a cation with formula H+
2. It consists of two hydrogen nuclei (protons), each sharing a single electron. It is the simplest molecular ion.
The LeRoy radius, derived by Robert J. LeRoy, defines the internuclear distance between two atoms at which LeRoy-Bernstein theory becomes valid.
In physics and chemistry, a degree of freedom is an independent physical parameter in the formal description of the state of a physical system. The set of all states of a system is known as the system's phase space, and the degrees of freedom of the system are the dimensions of the phase space.
Vibronic spectroscopy is a branch of molecular spectroscopy concerned with vibronic transitions: the simultaneous changes in electronic and vibrational energy levels of a molecule due to the absorption or emission of a photon of the appropriate energy. In the gas phase, vibronic transitions are accompanied by changes in rotational energy also.
Molecular symmetry in physics and chemistry describes the symmetry present in molecules and the classification of molecules according to their symmetry. Molecular symmetry is a fundamental concept in the application of Quantum Mechanics in physics and chemistry, for example it can be used to predict or explain many of a molecule's properties, such as its dipole moment and its allowed spectroscopic transitions, without doing the exact rigorous calculations. To do this it is necessary to classify the states of the molecule using the irreducible representations from the character table of the symmetry group of the molecule. Among all the molecular symmetries, diatomic molecules show some distinct features and they are relatively easier to analyze.
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