INTRODUCTION Analyses of the high-resolution solid-state 13 C NMR spectra of bulk polymers have revealed a sensitivity of the resonance frequen-cies observed to the local conformations of the chain fragment containing a particular carbon nucleus. 1 The connection between the resonance frequencies and local conformational environments is provided by the γ-gauche effect illustrated in Figure 1. For the all-carbon chain fragment examined there, it is noted that, when carbon nuclei o and γ are gauche to each other, they experience an ∼5 ppm shielding from the applied magnetic field, which is absent when they are further removed in their trans conformational arrangement. Whether polymers are constrained in their crystals to adopt a single rigid conformation or are molten, mobile, and free to inter-convert rapidly on the megahertz time scale between conformations, it has been repeatedly demonstrated 1À10 that the resonance fre-quencies observed for their carbon nuclei in high resolution solid-state 13 C NMR spectra depend principally upon and can be analyzed by means of the conformationally sensitive γ-gauche effect. The conformations and resonance frequencies (in ppm vs TMS) of the central methylene carbons in the butylene glycol fragments of poly(butylene terephthalate) (PBT) and several of its model compounds, as observed in their crystals, are presented in Figure 2. It has been shown that the central methylene car-bons in the butylene glycol fragment that are gauche to their γ-substituent ester oxygens are shielded (Figure 2) and resonate (3À4 ppm) upfield from those that are in a trans arrangement, consistent with the γ-gauche effect. PBT is a particularly relevant example, because here we compare the high-resolution solid-state 13 C NMR spectra and conformations of the closely related aromatic polyester poly(trimethylene terephthalate) (PTT) whose structure is shown in Figure 3. ' EXPERIMENTAL SECTION High-resolution solid-state 13 C NMR experiments were carried out on two different spectrometers: at 50 MHz on a Chemagnetics CMX-200S (PTT) and at 75 MHz using a Bruker DSX-300 [trimethylene glycol dibenzoate and the solid cyclic dimer of PTT (TMGDB and TT-2, respectively)], both with cross-polarization and with magic angle spinning (CP/MAS), and with high-power proton dipolar decoupling (DD) at spinning speeds ranging from 3 to 8 kHz. 13 C chemical shifts were referenced relative to TMS. CMX-200S spectra were obtained with 1000 transients, 1.0 ms contact time, and 3.0 s pulse delay. The spectral width was 15 kHz in 2K data points, which were zero-filled to 8K before Fourier transformation. DSX-300 magic-angle spinning (MAS) NMR measure-ments for the solid and melt states of TMGDB, and for the solid cyclic PTT dimer TT-2, were collected with a 4 mm double-resonance MAS probe. For the melt-state measurements, the probe temperature was calibrated using PbNO 3 to within (1 K. Comparisons of single-pulse 13 C spectra, acquired with high-power 1 H decoupling, to cross-polarization (CP) spectra acquired with a 1 ms contact time and 1 H decoupling, revealed no differences in isotropic chemical shift positions for either TMGB or TT-2.