Neutron Vibrational Spectroscopy Gives New Insights into the Structure of Poly(p-phenylene terephthalamide) M. Plazanet, † F. Fontaine-Vive, †,‡ K. H. Gardner, § V. T. Forsyth, †,| A. Ivanov, † A. J. Ramirez-Cuesta, ⊥ and M. R. Johnson* ,† Contribution from the Institut Laue LangeVin, BP156, 38042 Grenoble Cedex 9, France, Radiation, Reactors and Radionuclides Department, Faculty of Applied Sciences, Delft UniVersity of Technology, Mekelweg 15, 2629 JB Delft, The Netherlands, Department of Material Science and Engineering, UniVersity of Delaware, Newark, Delaware 19716, Lennard Jones Laboratory, School of Chemistry and Physics, Keele UniVersity, Staffordshire, ST5 5BG, UK, and ISIS, Rutherford Appleton Laboratory, Chilton, Didcot OX11 OQX, UK Received October 15, 2004; E-mail: johnson@ill.fr Abstract: The vibrational spectra of benzanilide and poly(p-phenylene terephthalamide) have been measured using inelastic neutron scattering. These compounds have similar hydrogen-bond networks, which, for poly(p-phenylene terephthalamide), lead to two-dimensional hydrogen-bonded sheets in the crystal. Experimental spectra are compared with solid-state, quantum chemical calculations based on density functional theory (DFT). Such “parameter-free” calculations allow the structure-dynamics relation in this type of compound to be quantified, which is demonstrated here for benzanilide. In the case of poly(p- phenylene terephthalamide), vibrational spectroscopy and DFT calculations help resolve long-standing questions about the packing of hydrogen-bonded sheets in the solid state. I. Introduction In materials lacking long-range structural order, vibrational spectroscopy is a powerful method for obtaining structural information. Diffraction techniques rely on long-range order to give well-defined Bragg reflections, whereas the vibrational modes of molecules depend on the local intra- and intermo- lecular environment. Intermolecular interactions typically extend over distances of tens of angstroms, but the effective range of interactions that determine vibrational modes in organic com- pounds is more like 5-6 Å. The structure-dynamics relation can be established empirically as is the case for determining the presence of secondary structures such as -sheets and helices in proteins from the optical, spectral profiles of the amide bands. 1,2 Indeed the study of -sheets has assumed greater importance over the past decade since highly aggregated -sheets have been established as symptoms of amyloid and prion diseases. However, a higher level of structural information is potentially available if a quantitative link can be made between structure and vibrations via accurate interatomic potentials. In the case of polypeptides, first principles-based calculations of spectral profiles have been employed, although these have been limited to the amide bands. 3,4 One of the original uses of vibrational spectroscopy was precisely that of parametrizing interatomic interactions. Typi- cally spectral frequencies rather than intensities were used for this purpose because, for optical spectroscopies (IR and Raman), spectral intensities have been difficult to determine, a knowledge of dipole moments and polarizability, respectively, being required. Although these quantities for optical spectroscopy can now be extracted from quantum chemical calculations, in inelastic neutron scattering (INS) spectral intensities are directly related to vibrational amplitudes via tabulated scattering cross- sections. The information content of INS spectra is more easily and reliably exploited. The quantitative link between structure and dynamics depends on the accuracy of the interatomic potentials that are used to calculate either the vibrational density of states from molecular dynamics simulations or the normal modes from the dynamical matrix. For smaller systems (less than a few hundred atoms), the second approach entails fewer calculations and gives directly the vibrational modes, including their wavevector dependence (phonon dispersion). This approach has been exploited in recent years to study hydrogen-bonded molecular crystals, using “parameter-free”, solid-state, density functional theory (DFT)- based methods to determine the interatomic force constants. 5-9 † Institut Laue Langevin. ‡ Delft University of Technology. § University of Delaware. | Keele University. ⊥ Rutherford Appleton Laboratory. (1) Kneipp, J.; Miller, L. M.; Joncic, M.; Kittel, M.; Lasch, P.; Naumann, D. Biochim. Biophys. Acta 2003, 1639, 152-158. (2) McColl, I. H.; Blanch, E. W.; Gill, A. C.; Rhie, A. G. O.; Ritchie, M. A.; Hecht. L.; Nielsen, K.; Barron, L. D. J. Am. Chem. Soc. 2003, 125, 10019- 10026. (3) Kubelka, J.; Keiderling, T. A. J. Am. Chem. Soc. 2001, 123, 6142-6150. (4) Kubelka, J.; Keiderling, T. A. J. Am. Chem. Soc. 2001, 123, 12048-12058. (5) Kearley, G. J.; Johnson, M. R.; Plazanet, M.; Suard, E. J. Chem. Phys. 2001, 115, 2614-2620. (6) Plazanet, M.; Fukushima, N.; Johnson, M. R.; Horsewill, A. J.; Tromms- dorff, H.-P. J. Chem. Phys. 2001, 115, 3241-3248. (7) Plazanet, M.; Fukushima, N.; Johnson, M. R. Chem. Phys. 2002, 280, 53- 70. (8) Johnson, M. R.; Trommsdorff, H.-P. Chem. Phys. Lett. 2002, 364, 34-38. Published on Web 04/16/2005 6672 9 J. AM. CHEM. SOC. 2005, 127, 6672-6678 10.1021/ja0437205 CCC: $30.25 © 2005 American Chemical Society