Alan Benesi

Alan Benesi

Main Content

  • Director of the NMR Facility and Lecturer in Chemistry
Office:
008 Chemistry Building
University Park, PA 16802
Email:
(814) 865-0941

Education:

  1. B.A., University of California, Santa Cruz, 1971
  2. Ph.D., University of California, Berkeley, 1975

Selected Publications:

D. J. Aurentz, F. G. Vogt, K. T. Mueller, and A. J. Benesi, Multiple-Rotor-Cycle QPASS Pulse Sequences: Separation of Quadrupolar Spinning Sidebands with an Application to 139La NMR, J. of Magnetic Resonance, 138:320-325 (1999).

E. F. Rakiewicz, A. J. Benesi, M. W. Grutzeck, and S. Kwan, Determination of the State of Water in Hydrated Cement Phases Using Deuterium NMR Spectroscopy, J. Am .Chem. Soc., 120:6415-6416 (1998).

A. J. Benesi, C. J. Falzone, S. Banerjee, and G. K. Farber, NMR Assignments for the Aldopentoses, Carbohydrate Research, 258:27-33 (1994).

X. Tang and A. J. Benesi, A 13C Spin-Lattice Relaxation Study of the Effect of Substituents on Rigid-Body Rotational Diffusion in Methylene Chloride Solution and in the Solid State, J. of Physical Chemistry, 98:2844-2847 (1994).

C. J. Falzone, A. J. Benesi, and J. T. J. Lecomte, Characterization of Taxol in Methylene Chloride by NMR Spectroscopy, Tetrahedron Letters, 13:3403-3413 (1992).

Information:

High-resolution and solid-state NMR experiments and theory; the role of water in hardening of hydrated cement phases.

Theoretical and Experimental NMR Spectroscopy

Nuclear magnetic resonance spectroscopy is the most powerful method available to characterize microscopic structure and dynamics at the atomic level. For a given magnetic field, the choice of NMR Larmor frequency determines the element and isotope in the Periodic Table that will be observed. Each peak or resonance in the NMR spectrum corresponds to a different environment for the nucleus. Every NMR spectrum provides convincing proof of atomic theory. Only X-ray crystallography approaches NMR in its ability to reveal the arrangement of atoms in matter, but unlike X-ray crystallography, NMR is not limited to crystalline substances with rigid atomic positions. NMR gives beautiful spectra for solids, liquids, and gases and can reveal the nature and rate of atomic motions in these phases.

In a paper published in 1998, Dr. Benesi and his collaborators used deuterium (2H) NMR to show that hardening of hydrated tricalcium silicate (the most important component of Portland cement) is associated with a phase change of the excess water from liquid to solid state. The deuterium nuclei within the 2H2O molecules undergo rapid tetrahedral jumps after hardening, similar to those observed in 2H2O ice just below the freezing point. The rapid tetrahedral jumps in the solid state give rise to a sharp liquid-like peak in the deuterium NMR spectrum that can only be distinguished from that of liquid 2H2O by means of its NMR relaxation parameters. For example, the T1 relaxation time of the peak decreases by over an order of magnitude during hardening. The change in T1 and the sharp liquid-like peak were both shown to be theoretically consistent with rapid tetrahedral jumps of the deuterium nuclei. Further support for this controversial hypothesis is provided by several other observations: The heat evolved during hardening is comparable to the heat of fusion of 2H2O, hardening takes six to eight times longer if 2H2O is used rather than 1H2O, and the spectrum expected for 2H2O ice is observed at sufficiently low temperatures (below -100 °C). Many questions still must be answered. Is a state change of excess water from liquid to solid the basis for hardening of all hydrated cements? What molecular species are directly associated with the excess water as it solidifies? Are the principal players hydrates or clathrates?

Peaks in liquid state NMR spectra are usually sharp lines, but peaks in solid state NMR are often broad and strangely shaped. Figure 1 shows the deuterium NMR spectrum of the 2H2O-hydrated pure cement phase Ca3Al2O6 one hour after addition of the 2H2O. In this case there are four different types of deuterium evident in the spectrum, a sharp central peak and three superimposed powder patterns. The powder patterns arise because the deuterium frequency for each type of relatively immobile deuterium varies as 3 cos2θ-1 where θ is the latitudinal angle made by the deuterium bonds with respect to the magnetic field Bo. In this case, the sharp central peak is due to excess 2H2O in the solid state with the deuterium nuclei experiencing rapid tetrahedral jumps which produces a sharp liquid-like peak. The narrowest powder pattern with "horns" at ±11.6 kHz is tentatively assigned to "tethered" deuterium undergoing rotational diffusion in a cone. The medium width powder pattern with "horns" at ±74 kHz is assigned to rigid 2H2O in the hexahydrate Ca3Al2O6•62H2O. The widest powder pattern with horns at ±98 kHz corresponds to Ca(O2H)2.

Dr. Benesi and his collaborators have also used liquid state NMR experiments to characterize a wide variety of important and interesting molecules. For example, they used 2D liquid state NMR methods to assign all of the 1H and 13C peaks for the anticancer drug taxol and all of the 1H and 13C peaks for the aldopentose sugars. In 1994, Dr. Benesi and his student used 13C NMR relaxation parameters to study the effects of substituents on rotational diffusion of nearly spherical adamantanes in the liquid and solid state.

Dr. Benesi is the Director of the Penn State NMR Facility. The goal of the facility is to help researchers use NMR to answer as many interesting questions in as many different areas as possible across the University. Graduate students who work with Dr. Benesi are expected to master both liquid and solid state NMR, and they will have the opportunity to collaborate on a wide variety of interesting research projects.