I recall trying to find this information about a year ago and having a lot of trouble finding it. Then I saw it published in my AOPA newsletter so I’m posting it here as a reminder to myself:
Question: What frequency, if any, can be used for air-to-air communications between two aircraft?
Answer: The Federal Communications Commission designates specific frequencies for aircraft and airports to use for specific purposes. For air-to-air communications, the designated frequencies are 122.75 MHz and 122.85 MHz. These frequencies also can be used for private airports that are not open to the public. For a list of designated frequencies for unicom and multicom frequencies, view Table 4-1-2 in the Aeronautical Information Manual. Learn more about what frequency you should use at AOPA Online (requires membership to AOPA).