You will find raspberry syrup in a number of older cocktail recipes such as the Clover Club, Albemarle Fizz and Bayard Fizz. For a period of time in the late 19th Century it seemed to be quite the rage. But raspberry syrup apparently fell out of favor as it seemed to disappear from drink recipes (or was replaced by grenadine) later in the 20th Century.
This is a shame because raspberry syrup has a lot to offer. It is not merely sweet and fruity but also offers a layer of dry acidity and "snap". Monin makes a pretty good one but it's really easy to make raspberry syrup at home.
Wash the berries and pat dry with a paper towel. Put the raspberries in a clean glass bowl or jar. Make a syrup from the water and sugar heating until the sugar has dissolved. Pour the syrup over the raspberries and cover. When cooled add the rum, stir and re-cover. Let sit 24 hours on the counter tightly covered.
Strain out the fruit the next day using successively smaller strainers. Start with a metal mesh strainer. Then move to a fine mesh strainer and finally a paper coffee filter that has been pre-wetted with water or syrup. The final straining will take awhile. Sometimes I just cover it with plastic wrap and let it strain overnight.
Once strained keep in a glass bottle. This recipes makes about 13-14 oz so a pint bottle or 375 mL liquor bottle are perfect. My raspberry syrup lasts many months in the refrigerator.
I usually make my fruit syrups at single strength (1:1 water to sugar). I increase the sugar a little here as the raspberries are a little more tart than other fruit. If you wish, you can add 1/8 to 1/4 tsp of citric acid to this recipe but I have found the raspberries provide enough acidity for me.
This is a fresher, "cold process" version of raspberry syrup. You can also simmer the raspberries in the syrup for awhile to get a more cooked syrup flavor.
Let me know how this turns out for you.