Comment on Orientational Distribution of Free O-H Groups of Interfacial Water is Exponential

In a recent letter (PRL,121,246101,2018), Sun et al. reported that combined MD simulation and sum frequency generation vibrational spectroscopy (SFG-VS) measurements led to conclusions of a broad and exponentially decaying orientational distribution, and the presence of the free O-H group pointing down to the bulk at the air/water interface. In this comment, we show that their main conclusions are based on questionable interpretation of the SFG-VS data presented in the letter [1], and are also contrary to the established data analysis and interpretations in the literature [2-5].