Solidstateprotonspin–latticerelaxationinfour structurallyrelatedorganicmolecules PeterA.Beckmann * ,KendraS.Burbank 1 ,MattyM.W.Lau 2 ,JessicaN.Ree 3 , TracyL.Weber 4 Department of Physics, Bryn Mawr College, Bryn Mawr, PA 19010-2899, USA Received2July2002 Abstract Wereportandinterpretthetemperaturedependenceoftheprotonspin–latticerelaxationrateat8.50and22.5MHz in four polycrystalline solids composed of structurally related molecules: 2-ethylanthracene, 2-t-butylanthracene, 2- ethylanthraquinone, and 2-t-butylanthraquinone. We have been unable to grow single crystals and therefore do not know the crystal structures. Hence, we use the NMR relaxometry data to make predictions about the solid state structures.Asexpected,weareabletoconcludethattheethylgroupsdonotreorientinthesolidstatebutthatthe t-butylgroupsdo.Theanthraquinoneshavea‘‘simpler’’structurethantheanthracenes.Thebestdynamicalmodels suggestthatthereisauniquecrystallographicsiteforthe t-butylgroupsin2-t-butylanthraquinoneandtwosites,each withhalfthemolecules,fortheethylgroupsin2-ethylanthraquinone.Therearealsotwositesin2-ethylanthracene,but withunequalweights,suggestingfoursitesintheunitcellwithlowersymmetrythanthetwoanthraquinones.Finally, the observed relaxation rate data in 2-t-butylanthracene is very complex and its interpretation demonstrates the uniquenessproblemthatarisesininterpretingrelaxometrydatawithouttheknowledgeofthecrystalstructure. Ó 2003ElsevierScienceB.V.Allrightsreserved. 1. Introduction Wereportlow-frequencynuclearmagneticres- onance(NMR)relaxometrymeasurementsinfour closely related Van der Waals organic molecular solids containing ethyl and t-butyl groups. We modelthemotionofthesealkylgroupsandusethe interpretation to investigate the relationship be- tween structure and motion. In addition, because theseintramolecularrotorsprobetheanisotropies in their local electronic environments, the results ofstudieslikethiscanbeusedtocommentonthe crystalpacking.Repeatedattemptstogrowsingle ChemicalPhysics290(2003)241–250 www.elsevier.com/locate/chemphys * Correspondingauthor. E-mail address: pbeckman@brynmawr.edu (P.A. Beck- mann). 1 Presentaddress:JeffersonLaboratory,HarvardUniversity, 17OxfordStreet,Cambridge,MA02138,USA. 2 Present address: College of Education, Science Teaching Center,2226H.R.W.BenjaminBuilding,UniversityofMary- land,CollegePark,MD20742,USA. 3 Present address: RiskMetrics Group, 44 Wall St., New York,NY10005,USA. 4 Present address: New York City MayorÕs Office, Office to Combat Domestic Violence, 100 Gold Street, 2nd floor, New York,NY10038,USA. 0301-0104/03/$-seefrontmatter Ó 2003ElsevierScienceB.V.Allrightsreserved. doi:10.1016/S0301-0104(03)00145-9