Bulk H 2 O differs structurally from H 2 O stabilized within the pores of a zeolite. Figure shows IR spectra of bulk water (Fig. ) and of water within Ti-FAU-OH (Fig. ), Ti-BEA-OH (Fig. ), Ti-MFI-OH (Fig. ), and Si-CDO-OH (Fig. ) at 313 K. These spectra show distinct features between 2750 -3750 cm -1 that reflect ν(O-H) of H 2 O molecules within chemically distinguishable bonding configurations. Experimental IR, Raman, and sum-frequency generation spectroscopies and computational predictions demonstrate that condensed-phase H 2 O molecules possess five specific hydrogen-bonding configurations (top of Fig. ; Table ) that include "free" H 2 O that lacks HB (Free; ~3636 cm -1 ), H 2 O that donates two and accepts one HB (DDA; ~3570 cm -1 ), H 2 O that donates one and accepts one HB (DA; ~3470 cm -1 ), H 2 O that both donates and accepts two HB (DDAA; ~3320 cm -1 ), and H 2 O that donates one and accepts two HB (DAA; ~3040 cm -1 ). The vibrational spectra of bulk H 2 O (Fig. ) shows that ν(O-H) of H 2 O in a DDAA bonding configuration dominates the measured spectra, which agrees with prior reports. Within the bulk-fluid phase, H 2 O molecules form dynamic three-dimensional hydrogen bonding structures described well by our MD simulations of TIP5P H 2 O (Fig. ). IR spectra of H 2 O within Ti-FAU-OH (Fig. ) show that H 2 O within the 1.3 nm supercages of hydrophilic FAU and of hydrophobic (pristine) Ti-FAU-F (Fig. ) appear largely similar and possess lineshapes that resemble bulk H 2 O. H 2 O within these supercages possess the spatial freedom to form threedimensional structures (Figs. and); yet, a fraction of H 2 O molecules adopt a DDAA configuration by simultaneously forming HBs with both SiOH defects and other H 2 O. These differences are reflected between the time-averaged spatial distributions of H 2 O in Ti-FAU-F and Ti-FAU-OH: the former exhibits a regular and well-defined boundary (Fig. ) whereas that latter contains irregularities caused by the attractive interactions between H 2 O and randomly sited silanol groups within the FAU structure (Fig. ). Within Ti-BEA-OH and Ti-MFI-OH, H 2 O molecules within the DA configuration account for a much larger fraction of the vibrational spectra (Figs. and). These hydrophilic materials, however, also stabilize greater numbers of DDAA H 2 O molecules than the analogous Ti-BEA-F (Fig. ) and Ti-MFI-F (Fig. ) materials, because H 2 O hydrogen bonds with (SiOH) x defects at the pore walls. Within Ti-BEA-OH, H 2 O molecules with the DDAA configuration congregate at intersections between pores (Figs. 1h, 1l, S4d, and S4g) or at mesoporous grain-boundary defects. In comparison to frameworks with larger pore diameter, H 2 O molecules within Si-CDO-OH (Fig. ) largely exist within the DA hydrogen bonding configuration. Collectively, the combined interpretation of these results suggest that H 2 O molecules form solvent structures that, on average, possess fewer hydrogen bonds per H 2 O molecule and different hydrogen bonding partners as the characteristic diameter of the surrounding pore decreases. To further investigate how topology and surface chemistry of the pores affect the structure of H 2 O, we compared the average number of HBs calculated from vibrational spectra (<N HB > IR ) to those observed during the MD simulation (<N HB > MD ). The experimental measurements and MD simulations show consistent differences among hydrophobic (Ti-zeolite-F) and hydrophilic (Ti-zeolite-OH) zeolite structures. The average number of HB decreases uniformly with the mean diameter of the zeolite pore and with the number of hydrogen bond donating residues on the pore walls. Figure establishes these trends via the level of parity between values of <N HB > IR and <N HB > MD across all materials examined. H 2 O within the bulk fluid phase and Ti-FAU possess an average of 3 -3.6 HBs irrespective of density of (SiOH) 4 . The average number of hydrogen bonds decreases monotonically towards an average value of ~2 as the characteristic pore diameter decreases across the series of Ti-BEA, Ti-MFI, and Si-CDO, because these sub-nanometer siloxane pores do not permit three-dimensional HB networks among H 2 O molecules. Further, values of <N HB > IR and <N HB > MD agree closely with prior reports, where MD simulations of defect-free Si-FAU, Si-MFI, and H + -form Al-BEA show that, on average, H 2 O within the FAU, BEA, and MFI frameworks possess ~3, ~2.7, and ~2 HBs per H 2 O molecule, respectively.