Students often face difficulties when presented with chemical structures and asked to relate them to properties of those substances. Learning to relate structures to properties, both in predicting properties based on chemical structures and interpreting properties to infer structure, is pivotal in students' education in chemistry. This troublesome but critical concept is often referred to as structure-property relationships. While there is no shortage of literature on students' difficulties with this concept, there is a lack of methodologies that can directly and quantitatively reveal underlying assumptions about structure-property relationships that constrain students' thinking. This study applied a "chemical thinking" lens to elucidate assumptions about structure-property relationships thinking. A combination of qualitative analysis using a think-aloud interview protocol was used with quantitative analysis of eye tracking data to probe students' reasoning when relating molecular structures of volatile hydrochlorocarbons to infrared spectral properties. Our initial findings offer partial validation of a newly developed methodology for analyzing eye tracking data to expose reasoning patterns that appear to correspond to identifiable underlying assumptions.