Help: How to analyze this NMR spectrum and determine the structure of the compound?

Analyzing an NMR spectrum to determine a molecular structure is a systematic process of integrating data from chemical shifts, integration values, coupling patterns, and often complementary techniques. The first step is to calculate the degree of unsaturation from the molecular formula, which provides immediate constraints on possible rings and pi-bonds. Without a formula, the total proton count from integration offers a starting point. You then assess the 1H NMR spectrum by segregating signals into regions: aliphatic protons (0–2 ppm), protons on carbons adjacent to electronegative atoms or pi-systems (2–4.5 ppm), and aromatic/alkene protons (5–8 ppm). The integration for each signal set reveals the number of equivalent protons, building discrete structural fragments. Critical analysis of multiplicity—singlets, doublets, triplets, or complex patterns—reveals the number of neighboring protons via the n+1 rule, allowing you to connect these fragments. For instance, a triplet indicates two neighboring protons, suggesting a CH2 group adjacent to a CH2 group, while a doublet suggests a CH group adjacent to a CH group.

The real analytical power emerges from synthesizing 1H data with 13C NMR and DEPT spectra when available. The 13C spectrum indicates the number of unique carbon environments, with signals in characteristic regions for sp3, sp2, and carbonyl carbons. A DEPT-135 experiment conclusively identifies methyl (CH3) and methine (CH) carbons as positive signals, methylene (CH2) carbons as negative signals, and quaternary carbons as absent, directly informing the hydrogenation state of each carbon. This data must be reconciled with the proton count and shifts. For example, a 13C signal around 180 ppm indicates a carbonyl, whose type (aldehyde, ketone, ester) is inferred from the absence of an aldehyde proton near 10 ppm or the presence of an adjacent O-CH3 group. Discrepancies, such as more carbon signals than expected, often point to symmetry elements within the molecule.

Structural elucidation proceeds by assembling fragments that account for all atoms, guided by the degree of unsaturation and the coupling networks. Long-range couplings or correlations from techniques like COSY (revealing protons on adjacent carbons) and HSQC (directly pairing each proton with its carbon) are used to connect fragments. The final proposed structure must satisfy every spectral datum: every chemical shift must be rationalized within the local electronic environment, every coupling constant must correspond to a plausible dihedral angle, and the integration must sum to the correct proton count. A critical validation step is predicting the spectrum of the proposed structure and checking for inconsistencies, such as an unexplained signal or a missing splitting. This iterative, puzzle-solving approach, moving from isolated spin systems to a complete constitutional structure, is the core mechanism of NMR analysis.