JOURNAL OF MATERIALS SCIENCE 37 (2 0 0 2 ) 3589 – 3594 Calculated NMR chemical shifts of nylon 6: a comparison of the α and γ forms J. CASANOVAS ∗ Departament de Qu´ımica, Escola Universit` aria Polit ` ecnica, Universitat de Lleida, c/Jaume II, n ◦ 69, Lleida 25001, Spain E-mail: jcasanovas@quimica.udl.es C. ALEM ´ AN ∗ Departament d’Enginyeria Qu´ ımica, E.T.S. d’Enginyers Industrials de Barcelona, Universitat Polit ` ecnica de Catalunya, Diagonal 647, Barcelona E-08028, Spain E-mail: carlos.aleman@upc.es The 13 C and 15 N chemical shifts of the α and γ crystal forms of nylon 6 have been estimated using ab initio calculations (GIAO method) with the B3LYP Density Functional and the 6-311G(d) basis set. Calculations were applied on proper model systems and the agreement with experimental data was quite good in all cases. Comparison between the results obtained for isolated chains and hydrogen bonded sheets allowed to elucidate the influence of the conformation and packing forces in the chemical shifts. These results open up new opportunities for the future use of quantum mechanical calculations as a complementary tool in the determination of the crystal structure of polymers. C  2002 Kluwer Academic Publishers 1. Introduction Nylon 6 is an important engineering plastic that has been subject of study from many researches [1–3]. Thus, a large number of papers dealing with its chemical synthesis, structural properties and industrial aspects have been reported during the last five decades. The two latter points are closely related since it was found that physical properties of the nylon 6 fibers depend on its crystalline structure [4, 5]. However, some questions related with the crystalline structure of nylon 6 remain unanswered. The two most common crystalline structures found in nylon 6 are those named α and γ , the former be- ing thermodynamically the most stable. In both forms the molecular chains are arranged in sheets, which are packed side-by-side, with stabilizing hydrogen bonds running in a single directions. Molecular conformations are however, different; so in the α form the chains are fully extended, whereas in the γ form the methylene next to the amide groups adopts a skew conformation, in a way similar to the pleated sheet of proteins. This change produces two distinctive features in the γ form: (i) the repeat unit length is shortened (about 0.35 ˚ A per amide group); and (ii) the amide groups are tilted (about 60 ◦ ) toward the planes which contain the methy- lene segment of the chain. Consequently, the hydrogen bonding direction is modified and a monoclinic pack- ing, which is usually described as pseudohexagonal by its peculiar dimensions, is preferred. ∗ Authors to whom all correspondence should be addressed. The α and γ crystal forms of nylon 6 have been widely investigated and distinguished by X-ray diffrac- tion [6]. Theoretical investigations based on energy cal- culations and/or simulations of the X-ray diffractions patterns have also been pursued [7]. On the other hand, the crystal structures of nylon 6 have been studied by both 13 C and 15 N Cross Polarization/Magic Angle Spin- ning (CP/MAS) NMR spectroscopy [5, 8]. Neverthe- less, no theoretical study about the solid-state NMR spectra has been reported until now. In this work we use the GIAO (Gauche Invariant Atomic Orbitals) method to analyze the NMR spectra of the two crystal forms of nylon 6. Thus, shielding properties of the atoms in a magnetic field have been computed by means of ab initio quantum mechanical techniques that include electron correlation effects. It should be noted that NMR is a local method insensitive to long range order, but sensitive to both the molecular conforma- tion and the close environment. Results have provided a quantitative and comprehensive understanding of the influence of both intra- and inter-molecular geometric parameters on the computed chemical shifts. 2. Methods 2.1. Molecular models There is a substantial confusion regarding the crys- talline structures of nylon 6 since different unit cells have been reported for both the α and γ forms [9]. However, this variability was explained by Parker and 0022–2461 C  2002 Kluwer Academic Publishers 3589