Site-Specific Assignments of C–H and C–D Vibrations in Gaseous 1-Butanol by High-Resolution Cavity-Enhanced Raman Spectroscopy

C-H and C-D vibrations serve as versatile Raman probes for molecular detection and structural characterization, while site-specific vibrational analysis remains challenging due to overlapping modes and complex isotope effects. 1-Butanol (CH₃CH₂CH₂CH₂OH), a model small molecule with four distinct C-H moieties along its carbon chain, offers an ideal platform to decipher such complexity─yet the assignment of its gas-phase vibrational spectra (including Fermi resonances and site-dependent modes) has long been hindered by insufficient spectral resolution. Using a sensitive cavity-enhanced Raman instrument developed recently, we recorded high-resolution gas-phase Raman spectra of 1-butanol and two selectively deuterated isotopologues (CH₃CD₂CD₂CD₂OH and CD₃CD₂CD₂CH₂OH) in the ranges of 900-3100 cm^-1, covering both C-H/C-D bending and stretching regions. By integration of quantum chemical calculations, isotope substitution, and polarization-dependent measurements, the spectral ambiguities were unraveled. Our analysis enables the assignments of all major spectral features, elucidating the role of symmetric and antisymmetric stretching vibrations and Fermi-resonant modes at each C-H site along the 1-butanol carbon chain. A systematic comparison of C-H versus C-D vibrational patterns allows us to quantify isotope-induced shifts in frequency and intensity. These findings not only advance fundamental understanding of 1-butanol's vibrational landscape but also provide a robust framework for site-specific Raman analysis of complex organic molecules and guide the design of Raman-based imaging probes for biological and environmental applications.