Table 2. Assignment of SFG Features and to OH Types and Their Orientation and H-Bonding Class.

frequency (cm–1) centered at	OH type	orientation	HB class
Al-terminated
∼3700	dangling surface aluminols (AlOH) formed from 2-up, and dangling OH in (1-up, 1-down) perpendicular to surface	2-up, (1-up, 1-down): OH up	2D in 2-up
			3D, 4D in (1-up, 1-down)
∼3450	OH in water molecules, hydrogen-bonded with other water molecules	(1-up, 1-down), 2-down	3D, 4D, 5T
∼3000 (broad peak)	delocalized protons	2-up	2D, 3D
∼2500 (broad peak)	delocalized protons	(1-up, 1-down): OH down	3D, 4D
O-terminated
∼3150	OH in water molecules, hydrogen-bonded	(1-up, 1-down), 2-down	3D, 4D, 5D