We introduce a broadly applicable technique to create nuclear spin singlet states in organic molecules and other many-atom systems. We employ a novel pulse sequence to produce a spin-lock induced crossing (SLIC) of the spin singlet and triplet energy levels, which enables triplet/singlet polarization transfer and singlet state preparation. We demonstrate the utility of the SLIC method by producing a long-lived nuclear spin singlet state on two strongly-coupled proton pairs in the tripeptide molecule phenylalanine-glycine-glycine dissolved in D2O, and by using SLIC to measure the J-couplings, chemical shift differences, and singlet lifetimes of the proton pairs. We show that SLIC is more efficient at creating nearly-equivalent nuclear spin singlet states than previous pulse sequence techniques, especially when triplet/singlet polarization transfer occurs on the same timescale as spin-lattice relaxation.There is great current interest in the controlled preparation and coherent manipulation of singlet states for nuclear spin pairs in molecules and other many-atom systems (e.g., spin networks in solids), as spin singlet states are largely decoupled from environmental perturbations that limit the spin state lifetime. For example, in liquid state experiments singlet states in nuclear spin pairs can exhibit lifetimes much longer than the single-spin polarization lifetime (T 1 ) [1][2][3][4][5][6][7][8][9][10][11][12]. In addition, nuclear spin singlet states can be used as a resource for spectroscopic interrogation of couplings within many-spin systems, including J-couplings, dipolar, and hyperfine couplings in both organic molecules and spin networks in solids [13][14][15]. Such singlet states exist naturally when nuclear spins are strongly J-coupled relative to their resonance frequency differences, ∆ν, i.e., J >> ∆ν. However, due to the differences in spin singlet and triplet state symmetries, it is not possible to transfer polarization from the triplet to the singlet state by directly driving a radiofrequency transition, which limits the control of singlet state preparation and manipulation. Tayler and Levitt demonstrated that triplet/singlet polarization transfer can instead be acheived using a series of π-pulse trains in which the pulse timing is synchronized to the J-coupling strength between nuclei [5]. This "M2S" sequence takes advantage of the small amount of mixing between singlet and triplet states that is present when- * devience@fas.harvard.edu † rwalsworth@cfa.harvard.edu ‡ mrosen@cfa.harvard.edu ever ∆ν > 0. Feng and Warren also showed that the M2S sequence can create singlet states in certain heteronuclear systems even when ∆ν = 0 [12]. These results hold promise for creating hyperpolarized singlet states without the need for a symmetry-breaking chemical reaction or continuous spin-locking [11]. However, in all results to date, the polarization transfer to the spin singlet state only occurs during the final third of the M2S sequence time, and before this stage the spin polarization occupies states subj...